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In narrow houses, the return-pipes may be placed upon the side walls, but as the width increases it will be generally advisable to have from one-third to one- half of the returns either under the benches or in the walks when beds are used. From the fact that running the pipes overhead will not only improve the circulation but will prevent cold draughts of air upon the plants, it is often desirable when but one overhead flow-pipe is used to bring back one return upon each of the purlins. When the end of the house is much exposed, it is an excellent plan to drop down one feed - pipe from the end of the main, or two when there is a door in the end of the house, and supply coils running in either direction to the corner of the house and thence along the walls toward the end where the heater is located. Particularly when the pipes are but little, if any, above the top of the boiler, the circulation will be improved by carrying the return-pipes as high as possible, but of course care should be taken when they are under the benches not to have them so high that they will dry out the soil.
In narrow houses, the return-pipes may be placed upon the side walls, but as the width increases it will be generally advisable to have from one-third to one- half of the returns either under the benches or in the walks when beds are used. From the fact that running the pipes overhead will not only improve the circulation but will prevent cold draughts of air upon the plants, it is often desirable when but one overhead flow-pipe is used to bring back one return upon each of the purlins. When the end of the house is much exposed, it is an excellent plan to drop down one feed - pipe from the end of the main, or two when there is a door in the end of the house, and supply coils running in either direction to the corner of the house and thence along the walls toward the end where the heater is located. Particularly when the pipes are but little, if any, above the top of the boiler, the circulation will be improved by carrying the return-pipes as high as possible, but of course care should be taken when they are under the benches not to have them so high that they will dry out the soil.
The returns may be arranged in horizontal coils under the benches, or in vertical coils on the walls, or on the sides and supports of the beds and benches. The pipes in the coils may be connected at their ends either by means of manifolds, or by tees and close nipples, but in either case provision should be made for
The returns may be arranged in horizontal coils under the benches, or in vertical coils on the walls, or on the sides and supports of the beds and benches. The pipes in the coils may be connected at their ends either by means of manifolds, or by tees and close nipples, but in either case provision should be made for expansion of the pipe which with vertical wall coils may be done by running them partly across the ends of the houses and the same means may be used in horizontal coils, or the headers at the lower ends of the coils may be connected with the ends of the pipes by means of nipples and right and left ells. Whenever possible, there should be at least two returns supplied by each of the flow-pipes and the number may be increased until the capacity of the flow is reached. In determining just how many returns may be supplied by a given flow- pipe, one should always make allowance for the radiation furnished by the flow-pipe itself and, as the friction will be greater in a large number of short returns than for the same radiation with long returns, this should be considered in adjusting the ratio between the flow- and return-pipes.
Even greater attention should be given to the grading of the small return-pipes than to the larger flow-pipes, as the danger from pocketing of the air will be increased. For the smaller sizes, it will be advisable to give them a slope of at least 1 inch in 15 feet; but, if carefully graded and securely supported at intervals of 10 feet, good results can be obtained with 2-inch pipe with a fall of 1 inch in 20 feet; and if no more than 1 inch in 30 feet is available even this light fall will generally suffice to rid the pipes of air. This is really the main object for which the pipes are sloped, as the circulation would be fully as good, or better, if they are run on a level from the highest point in the system, provided the air did not pocket.
By having the highest point in the system near the boiler and attaching the expansion-tank at that point, one secures a downhill arrangement of the pipes which not only gives a better circulation than when the flow-pipes run uphill, but it does away entirely with air- valves which must be provided when the flow-pipe runs uphill and which often give trouble.
The method of piping which has been advocated, i.e. running one or more pipes in each house to the farther end and there connecting them with the returns, will give a more even temperature than can be secured in any other way. Formerly, it was the custom to connect the supply-pipes with the coils at the end of the house nearest the boiler. In some cases, one-half of the pipes in the coils served as flows to feed an equal number of return-pipes, or all of the pipes in the coil were connected at the farther end of the house with a main return-pipe, of the same size as the feed-pipe, which was brought back underneath the coil, or all of the coils in the house were connected into one main return. When the latter arrangement is used, the heating of the house is less uniform than with an overhead flow-pipe, the farther end of the house being cooler than the one near the heater.
Unless the heating system is connected directly with the water-supply system, which is used as an expansion- tank, a special tank must be provided and connected with the highest part of the flow-pipe or with one of the returns near the heater. While it would answer if this tank is located at some point but slightly above the heating system, it is always desirable to have it somewhat elevated, as this will raise the boiling-point of the water in the system and hence increase its efficiency, as well as lessening the danger of its boiling over. The pipe connecting the expansion-tank with the heating-pipes should not be less than ¾ inch and this should be increased to 1 ½ to 2 inches in large systems. The size of the expansion-tank should be sufficient to equal the amount which the water in the system will increase in volume when it is raised from a temperature of 40° to 200°, with a margin of perhaps 50 per cent. By connecting the expansion-tank with the highest part of the system, one not only does away with the necessity of using air-valves but also lessens the tendency of the water to boil over.
When there are several houses in the range connected with one system, it is always a good practice to have a valve upon the supply-pipe leading to each house, with other valves upon at least one-half of the coils. It will thus be possible to reduce the radiation in each house or to cut it out entirely if desired.
Hot water under pressure.
Especially in large ranges it is now becoming customary to place the water under pressure, thus making it possible to raise the temperature at which it will boil, and in this way the circulation can be improved, and instead of the water in the returns having an average temperature of 150°, it can be maintained several degrees above the ordinary boiling-point of water. The principal objection to this plan is that the water in the boiler being hotter, the gases of combustion are not cooled down to the same extent as when the water is at 160° or less. This results in lessening the economy of coal-consumption, placing it upon about the same plane as when steam is used. On the other hand, this system has the merit of reducing the amount of radiation required in the heating - system, and in this way lessening the cost of piping the greenhouse fully twenty-five per cent.
Various methods of placing the water in the heating- system under pressure have been employed. Among them is to use a safety-valve and a vacuum-valve, either upon the expansion-tank, or if this is not closed, upon the expansion-pipe within the tank. The safety- valve allows either the air or the water, as the case may be, to pass out of the system when the pressure desired is reached, while the vacuum-valve permits the air or water to re-enter the system when the pressure drops.
What is known as the "mercury generator" or "circulator" also serves the same purpose. In these a column of mercury prevents the escape of the water in the system until the pressure has reached the point desired, when it allows a portion of the water to escape and, later on, to re-enter the system when the pressure decreases. It will be seen that this acts in exactly the same way as the safety-valve and vacuum-valve described above. By raising the boiling-point of water and improving the circulation, it not only makes it possible to use smaller pipes both for flows and returns, but the amount of radiation required will be considerably reduced. In fact, although it is not advisable to carry it to that extent, it is possible to reduce the amount of radiation practically to that required for steam-heating.
This system is of value particularly in sections of the country in which the usual winter temperature is well above zero but where the mercury drops 10° to 15° for a short period each winter. By piping the houses so that the desired temperature can be obtained for the houses in ordinary weather by using an open system, it will then be possible by using a "circulator" to maintain the same temperature in the houses even though the mercury drops 15° or 20° lower. This will make a considerable difference in the cost of piping the houses and the efficiency of the system so far as coal is concerned will be affected only during the few days when the use of the "circulator is necessary.
The use of a closed system is also helpful when, owing to local conditions, it is necessary to place the boiler upon or slightly below the level of the walks in the houses. While much can be done to secure a circulation by using overhead flows and keeping the returns as high as possible, the circulation can be still further improved if it is run as a closed system. Still another method of increasing the rapidity of the circulation and the efficiency of the heating-system is to place either upon the main flow- or return-pipe a pump, worked by steam or electricity, by which it will be possible greatly to accelerate the circulation of the water, so that such matters as the relative elevation of the boiler and heating- pipes will need but little consideration and it will be possible to decrease to a considerable extent the size and number of the heating-pipes.
Estimating hot-water radiation.
Owing to the great variations in temperature and the differences in the construction of greenhouses, and also in their exposures, it is impossible to give any explicit rules regarding the amount of radiation that will be required under all conditions; but experience has shown that in well-built houses any desired temperature can be secured. Knowing the minimum outside temperature and the temperature to be maintained within the house, it is necessary only to install a heating-plant with a radiating surface having a certain definite ratio to the amount of exposed glass and wall surface. It is, of course, understood that there must be a proper adjustment between the size of the boiler and the radiating surface and that the system is so arranged as to (five good results. Thus, when a temperature of 40° is desired in sections in which the mercury does not drop below zero, it will be possible to maintain it when 1 square foot of radiating surface is provided for each 5 square feet of glass; if 45° is required there should be 1 foot of radiation for 4 ½ feet of glass. Under the same conditions, 50°, 55°, 60°, 65° and 70° can be obtained, respectively, by using 1 square foot of radiating surface for each 4, 3½, 3,2½ , and 2 square feet of glass. When the outside temperatures are slightly under or above zero, there should be a proportionate increase or decrease in the amount of pipe used; and, if the houses are poorly constructed or an exposed location, it will be desirable to provide a still further increase in the amount of radiating surface. Under the very best conditions, the temperatures mentioned can be obtained with a slightly smaller amount of radiation, but the greatest economy so far as coal-consumption and labor are concerned will be secured when the amount of radiation recommended is used.
In determining the amount of exposed glass surface, the number of square feet in the roofs, ends and sides of the houses should be added, and to this it will be well to add one-fifth of the exposed wooden, concrete or brick wall surfaces. If the amount thus obtained is divided by the number which expresses the ratio between the area of glass and the amount of radiation which will be required, it will give the number of square feet of heating-pipe which must be installed. The unit of measurement of wrought pipe is its interior diameter, while its radiating surface is determined by its outside circumference, and, although it will vary slightly according to the thickness of the pipe, it is customary to estimate that 1-inch pipe will afford about .344 square feet of radiating surface to the linear foot, while 1 ¼ -, 1 ½ -, 2-, 2 ½ -, and 3-inch pipe will furnish respectively .434, .497, .621, .759 and .916 square feet of radiation for each foot in length of pipe. Tne following example will perhaps aid in determining the amount of radiating surface and its arrangement in a greenhouse. If a house is 32 feet in width and 200 feet in length, with 30 inches of glass in each side wall and with one end only of exposed glass, and a concrete wall 3 feet high on two sides and one end, there will be about 9,000 square feet of glass. To heat this to 50° in zero weather it will be necessary to use one-fourth as much radiating surface, or 2,250 square feet. In a house of this length it will be possible to supply this amount of radiation by means of five 2 ½ -inch flow-pipes, and the remaining radiation will be provided by means of ten 2-inch returns which will allow two for each of the flow-pipes. These figures are intended to apply when an open system is used but, if a "generator" is attached, not to exceed four flows and eight returns will be required.
The use of long, straight runs of pipe will give the best results and, whenever possible, ells and tees should be avoided, but if they must be employed special hot-water fittings should be secured.
In conservatories with high side walls it is desirable to place the flow-pipes at the plate and the returns on the walls or under the tables. Figs. 1764-1766 illustrate the lay-out of pipes in carnation-, rose-, and violet-houses.
Heating by flues.
When fuel is cheap, and when either a low temperature is desired in the house, or the outside temperature does not drop much below the freezing point, hot-air flues may be used but, while the cost of constructing them is small, the danger of fire is so great that they are often found to be far from economical. A brick furnace is built at one end of the house and from this a 10- or 12-inch flue is constructed to carry the smoke and hot gases through the house to the chimney which may be either at the farther end of the house or directly over the furnace, the flue, in the latter case, making a complete circuit of the house. When the houses to be heated are more than 60 feet long, it is advisable to have a furnace in each end, with the flue from each extending only to the center of the house and returning to the end from which it started. For the first 30 feet the lining of the flue, at least, should be of fire-brick, but beyond that the flue may be constructed of sewer- pipe.
Piping for steam.
Except that it is possible to use smaller flow- and return-pipes, the arrangement of the piping for steam- heating is not very unlike that described for hot water. Unless the houses are more than 30 feet wide and 150 feet in length, only one flow-pipe need be used and that can be carried from 2 to 4 feet below the ridge. In wider and longer houses, it is generally advisable to put in two or more flows. One of these flows can be carried on each wall-plate and in extremely wide houses others may be under the ridge and purlins.
For determining the size of steam mains, a good rule to use is to take one-tenth the square root of the radiation to be supplied and consider this to be the diameter in inches of the main required. Thus for supplying 400 square feet of radiating surface we would take one- tenth the square root of 400 (i/400-1-10 = 2), which will give 2 inches as the diameter of the main required. As the amount of radiation increases, a slight reduction can be made in the size of the mains and 2 ½ -, 3-, 3 ½ -, and 4-inch supply-pipes will answer respectively for 700, 1,000, 1,400 and 1,900 square feet of radiation. This is intended to apply with low-pressure steam, and as the steam-pressure is increased above five pounds a slight decrease in the size of the mains would be permissible.
The size of the pipes to be used for the coils will also depend upon the length of the house. For ordinary lengths 1 ¼ -inch pipe will be desirable, but, when they are more than 250 feet in length, 1 ½ -inch pipe may be used with low pressure steam and, in those much less than 100 feet, 1-inch pipe will answer. The location and arrangements of the coils will necessarily be determined by the openings in the walls and whether beds or raised benches are used. One of the simplest and most satisfactory ways of piping a greenhouse of moderate size, say from 20 to 30 feet in width and up to 150 feet in length, is to run the flow-pipe, which would be either 2- or 2 ½ -inch, overhead and bring back the coils on the walls, or, if raised benches are used and crops for which bottom heat will be helpful are to be grown, from one-third to one-half of the return- pipes may be distributed under the benches and the remainder may be on the walls. The return-coils should of course be given a slight slope toward the boiler, care being taken that no opportunity is afforded for the air to pocket and prevent the free flow of the water from the condensed steam back toward the boiler. A fall of 1 inch in 10 feet will suffice, and even less will answer if care is taken in grading and supporting the pipes.
In order to prevent the water from backing up in the coils, it is desirable that they should be at least 18 or 20 inches above the level of the water in the boiler, while 3 or 4 feet would be even better and will be necessary in large ranges. Unless this can be secured it will not be possible to return the water of condensation to the boiler by gravity and either a steam trap or pump should be provided for the purpose. By means of these, the water can be carried to a water feed-tank from which it can be fed into the boilers.
There should be an automatic air-valve at the end of each coil and, in order to regulate the amount of steam, a shut-off valve should be placed upon both flow- and return-pipes leading to each house. Unless there are several coils in each house, one or more of which could be cut off by means of valves, it will always be well to have valves upon a number of the pipes in the coils so that all but one or two can be cut off if desired. To prevent the water from being forced out from the boiler when the steam is turned into the houses, there should be a check valve in the main return-pipe near the boiler.
The amount of radiation which will be required to secure a given temperature will vary to some extent with the amount of pressure carried in the boiler, or in the coils, when a reducing-valve is used, but as a rule, this is not much more than five pounds and often it is even less. It will be best to provide a sufficient amount of radiation to furnish the temperature desired in ordinary cold weather without carrying any pressure and then, by raising the pressure to five to ten pounds, secure the heat required during the cold waves.
In determining the amount of radiation for a steam- heated house, for zero weather, it will answer if one considers that 1 square foot of pipe will heat 9 square feet of glass when 40° are desired, and will suffice for 7, 5 and 3 where 50°, 60°, and 70°, respectively, are, required. Fig. 1767 illustrates the piping required for heating a rose-house with steam. L. R. Taft.
Greenhouse management
Persons usually learn to grow plants under glass by rule of thumb. Such practical knowledge is always essential, but better and quicker results are secured if underlying truths or principles are learned at the same time. Even if no better results in plant-growing were to be attained, the learning of principles could never do harm, and it adds immensely to the intellectual satisfaction in the work. There is no American writing that essays to expound the principles of greenhouse management, although there are manuals giving direct advice for the growing of different classes of plants. There are two kinds of principles to apprehend in greenhouse management,—those relating to the management of the plains themselves, and those dealing primarily with the management of the house.
The first principle to be apprehended in the growing of plants under glass is this. Each plant has its own season of bloom. Every good gardener knows the times and seasons of his plants as he knows his alphabet, without knowing that he knows. Yet there are many failures because of lack of this knowledge, particularly among amateurs. The housewife is always asking how to make her wax-plant bloom, without knowing that it would bloom if she would let it alone in winter and let it grow in spring and summer. What we try to accomplish by means of fertilizers, forcing and other special practices may often be accomplished almost without effort if we know the natural season of the plant. Nearly all greenhouse plants are grown on this principle. We give them conditions as nearly normal to them as possible. We endeavor to accommodate our conditions to the plant, not our plant to the conditions. Some plants may be forced to bloom in abnormal seasons, as roses, carnations, lilies (see Forcing). But these forcing plants are few compared with the whole number of greenhouse species. The season of normal activity is the key to the whole problem of growing plants under glass; yet many a young man has served an apprenticeship, or has taken a course in an agricultural college, without learning this principle.
The second principle is like unto the first: Most plants demand a particular season of inactivity or rest. It is not rest in the sense of recuperation, but it is the habit or nature of the plant. For ages, most plants have been forced to cease their activities because of cold or dry. These habits are so fixed that they must be recognized when the plants are grown under glass. Some plants have no such definite seasons, and will grow more or less continuously, but these are the exceptions. Others may rest at almost any time of the year; but most plants have a definite season, and this season must be learned. In general, experience is the only guide as to whether a plant needs rest; but bulbs and tubers and thick rhizomes always signify that the plant was obliged, in its native haunts, to carry itself over an unpropitious season, and that a rest is very necessary, if not absolutely essential, under domestication. Instinctively; we let bulbous plants rest. They usually rest in our winter and bloom in our spring and summer, but some of them—of which some of the Cape bulbs, as nerines, are examples—rest in our summer and bloom in autumn.
The third principle from the plant side is this: The greater part of the growth should be made before the plant is expected to bloom. It is natural for a plant first to grow: then it blooms and makes its fruit. In the greater number of cases, these two great functions do not proceed simultaneously, at least not to their full degree. This principle is admirably illustrated in woody plants. The gardener always impresses on the apprentice the necessity of securing "well-ripened wood" of azaleas, camellias, and the like, if he would have good flowers. That is, the plant should have completed one cycle of its life before it begins another. From immature and sappy wood only poor bloom may be expected. This is true to a degree even in herbaceous plants. The vegetative stage or cycle may be made shorter or longer by smaller or larger pots, but the stage of rapid growth must be well passed before the best bloom is wanted. Fertilizer applied then will go to the production of flowers; but before that time it will make largely for the production of leaf and wood. The stronger and better the plant in its vegetative stage, according to its size, the more satisfactory it should be in its blooming stage.
Closely like the last principle is the experience that checking growth, so long as the plant remains healthy, induces fruitfulness or floriferousness. If the gardener continues to shift his plants into larger pots, he should not expect the best results in bloom. He shifts from pot to pot until the plant reaches the desired size; then he allows the roots to be confined, and the plant is set into bloom. Over-potting is a serious evil. When the blooming habit is once begun, he may apply liquid manure or other fertilizer if the plant needs it. The rose- grower or the cucumber-grower wants a shallow bench, that the plants may not run too much to vine.
A carnation-grower writes that there is "little difference in the yearly average as to quality or quantity of flowers, but plants grown on shallow benches come into flower more quickly in the fall. Those grown in solid beds produce an abundance of flowers later in the season. The preference of commercial carnation- growers is for raised benches so that there may be more blooms early in the fall and at the Christmas holidays."
The natural habitat of the plant is significant to the cultivator; it gives a suggestion of the treatment under which the plant will be likely to thrive. Unconsciously the plant-grower strives to imitate what he conceives to be the conditions, as to temperature, moisture and sunlight, under which the species grows in the wild. We have our tropical, temperate and cool houses. Yet, it must be remembered that the mere geography of a plant's native place does not always indicate what the precise nature of that place is. The plant in question may grow in some unusual site or exposure in its native wilds. In a general way, we expect that a plant coming from the Amazon needs a hothouse; but the details of altitude, exposure, moisture and sunlight must be learned by experience. Again, it is to be said that plants do not always grow where they would, but where they must. Many plants that inhabit swamps thrive well on dry lands.
Yet, the habitat and the zone give the hint: with this beginning, the grower may work out the proper treatment. Examples are many in which cultivators have slavishly followed the suggestion given by a plant's nativity, only to meet with partial failure. Because the dipladenia is Brazilian, it is usually supposed that it needs a hothouse, but it gives best results in- a coolhouse. Persons often make a similar mistake in growing the pepino warm, because it is Central and South American. Ixia is commonly regarded in the North as only a glasshouse subject because it is a Cape bulb, yet it thrives in the open in parts of New England, when well covered in winter.
The best method of propagation is to be determined for each species; but, as a rule, quicker results and stockier plants are secured from cuttings than from seeds. Of necessity, most greenhouse plants are grown from cuttings. In most cases, the best material for cuttings is the nearly ripe wood. In woody plants, as camellias and others, the cutting material often may be completely woody. In herbaceous plants, the proper material is stems which have begun to harden. Now and then better results are secured from seeds, even with perennials, as in grevillea and Impatiens sultani.
Coming, now, to some of the principles that underlie the proper management of the house, it may be said, first of all, that the grower should attempt to imitate a natural day. There should be the full complement of continuous sunlight; there should be periodicity in temperature. From the lowest temperature before dawn, there should be a gradual rise to midday or later. As a rule, the night temperature should be 10 to 15° F. below the maximum day temperature in the shade. A high night temperature makes the plants soft and tends to bring them to maturity too early. It makes weak stems and flabby flowers. The temperature should change gradually: violent fluctuations are inimical, particularly to plants grown at a high temperature.
In greenhouse cultivation, every plant is to receive individual care. In the field, the crop is the unit: there we deal with plants in the aggregate. In the greenhouse, each plant is to be saved and to receive special care: upon this success depends. There should be no vacant places on the greenhouse bench; room is too valuable. All this means that every care should be taken so to arrange the house that every plant will have a chance to develop to its utmost perfection. Patient hand labor pays with greenhouse plants. The work cannot be done by tools or by proxy. Therefore, the gardener becomes skilful.
Every caution should be taken to prevent the plants from becoming diseased or from being attacked by insects. The greater part of insect and fungous troubles in the greenhouse is the result of carelessness or of mistakes in the growing of the plants. Determine what diseases or pests are likely to attack any plant; discover under what conditions these diseases or pests are likely to thrive; then see that those conditions do not arise. Keep the house sweet and clean. Destroy the affected parts whenever practicable. Then if trouble come, apply the fungicide or the insecticide. Remember that the very protection which is given the plants, in the way of equable conditions, also protects their enemies: therefore, it is better to count on not having the difficulties than on curing them. If uncontrollable diseases or pests have been troublesome, make a complete change of soil or stock before the next season, if practicable. At least once every year there is an opportunity to rid the place of pests. Nematodes may be frozen out. Many gardeners carry their troubles year by year by trying to fight them, when they might succeed by trying to avoid them.
Of course, the greenhouse man must provide himself with the best insecticides and fungicides, and with good apparatus. The efficiency of these materials and appliances has greatly improved in recent years, and most of the old pests may now be controlled.
The higher the temperature and the more rapid the growth, the greater the care necessary to insure good results. Plants grown under such conditions are soft and juicy. They are easily injured by every untoward circumstance, particularly by drafts of cold air. Let a draft of cold air fall on cucumbers or rapid-growing roses, and mildew will result in spite of bordeaux mixture and brimstone.
In dark weather, grow the plants "slow." If given too much heat or too much water, they become soft and flabby, and fall prey to mildew, green-fly and other disorders. A stocky plant is always desirable, but particularly in the dull weather and short days of midwinter: at that time, extra precautions should be taken in the management of the house.
Watering plants under glass requires more judgment than any other single operation. Apply water when the plants need it, is a gardener's rule, but it is difficult to follow because one may not know when they need it. Yet if the gardener will put the emphasis on the word need he will at least be cautioned: novices often apply the advice as if it read: Apply water when the plants will stand it. Water thoroughly at each application. Mere dribbling may do more harm than good. Many persons water too frequently but not enough. Remember that in benches evaporation takes place from both top and bottom; and in pots it takes place from all sides. Water on a rising temperature. This advice is specially applicable to warmhouse stuff. Watering is a cooling process. The foliage should not go into the night wet, particularly if the plant is soft-growing or is a warmhouse subject. Water sparingly or not at att when evaporation is slight, as in dull weather.
In all greenhouse work, see that the soil is thoroughly comminuted and that it contains much sand or fiber. The amount of soil is small: see that it is all usable. In the garden, roots may wander if good soil is not at hand: in pots they cannot. The excessive watering in greenhouses tends to pack the soil, particularly if the water is applied from a hose. The earth tends to run together or to puddle. Therefore, it should contain little silt or clay. The practice of adding sand and leaf- mold to greenhouse soil is thus explained.
Ventilation is practised for the purpose of reducing temperature and of lessening atmospheric moisture. Theoretically, it is employed also for the purpose of introducing chemically fresh air, but with the opening and shutting of doors, and unavoidable leaks in the house, it is not necessary to give much thought to the introduction of mere fresh air. Ventilating reduces the temperature by letting out warm air and letting in cool air. The air should be admitted in small quantities and at the greatest distance from the plants in order to avoid the ill effects of drafts on the plants. Many small openings are better than a few very large ones. Ventilate on a rising temperature.
Most plants require shading in the summer under glass. Shading is of use in mitigating the heat as well as in tempering the light. A shaded house has more uniform conditions of temperature and moisture. If plants are grown soft and in partial shade, they are likely to be injured if exposed to bright sunlight. Sun- scalding is most common in spring, since the plants are not yet inured to bright sunshine and strong sun heat. The burning of plants is due to waves (not bubbles) in the glass. Other things being' equal, the larger the house the easier is the management of it. It is less subject to fluctuations of temperature and moisture. Greenhouses built against residences are specially liable to violent fluctuations; the body of air is small and responds to all external changes. L.H.B.
