Five systems of heating greenhouses have been in use. The oldest and simplest is the flue. Except in the propagation of sweet potato plants and in very small places where fuel and labor are cheap it has given place to one of the other four. Hot water under low pressure or in the open tank system is still used in small establishments such as private conservatories; water under high pressure or in practically closed circuits, as in larger but not the largest commercial places; low pressure steam or the so called vacuum or vapor system, more or less adaptable to any sized establishment; and high pressure steam, most popular in the very large greenhouse ventures. Low pressure hot water employs large cast iron pipes "leadeclo at the It is too cumbersome and too limited in its adaptability for commercial practices. In the other three systems smaller sized wrought iron pipes are screwed together and carried wherever desired beneath or above the benches. All three of these systems when properly in stalled give good results. \V1 rich one shall be chosen is largely a matter of individual establishment and personal The ()Yen tank hot water system more elastic than the others in its adaptability to general lay-outs and in the ease with which it may be managed. Once started, its operation is practically automatic for periods of 6 to 10 hours without attention. High pressure water and the steam systems demand much more con stant attention because the temperatures rise and fall far more quickly.
While the principles of greenhouse con struction have been made the study of skilled engineers and architects who confine their ef forts to this class of work greenhouse manage ment has from the beginning until the present been almost wholly by rule of thumb. Thus progress has been slower than had fundamental principles been understood and applied. These principles naturally divide themselves into two groups: those that apply to the plants them. selves and those that apply to the management of the greenhouse.
Among those principles of the former group the following are perhaps the more important: (1) Every species of plant has its own seas of flowering. Most failures to secure flowers are due to ignorance or disregard of this prin. ciple. By knowing the natural season of bloom and by supplying conditions as nearly natural as possible to the plant, success is almost sure to follow. The number of plants that can be made to blossom at abnormal times are few as compared with the whole number of species grown in greenhouses. (2) Most plants need a period of rest. This natural demand is due to the natural upbringing of each, due usually to the approach of weather too cold, too hot or too dry for the plant to continue action. Plants that do not need such rest are the ex ception, not the rule. (3) Most of the plant's growth should have been made before bloom should be expected. In other words the vege table cycle must be complete or at least far advanced before the reproductive one should be looked for. (4) A check in vegetative
growth, without impairment of health, induces blooming and fruitfulness. To illustrate: by frequent shifting of plants from smaller, to larger flower pots the blooming time may be delayed; but by stopping the shifting when 'the plants have attained the desired size flowers will soon appear. Once started into bloom plant food may be given as freely as the plant may need it. (5) The plant's habitat is a reliable index of the treatment needed. The more nearly the grower can imitate the natural home of the plant the more likely is he to succeed. (6) The best method of propagation must be learned for each species; usually, how ever, quicker results and sturdier plants will develop from cuttings than from seeds.
Among those principles that relate to the greenhouse the following take lead: (1) As nearly as possible conditions in the greenhouse should imitate a natural day — light full and continuous, temperature waxing toward and waning from midday with a minimum night registration of 10 to 15 degrees below the shade maximum during the day. (2) Each plant must receive individual attention. Space is ton costly to slight anything. (3) Prevent damage by foes—insects, fungi, bacteria, worms, etc. Preventive sanitation favors success. (4) High temperature and ample water make for rapid, sappy growth. Hence greater care is needed to prevent injury to the plants. Disease is more likely to attack such plants than those grown under lower temperature and with less water. (5) In cloudy weather plants naturally grow (Slowly. Therefore in the greenhouse reduce heat and water and give more ventilation. (6) -Water only when the plants need watering, but water copiously then. An occasional drench is far better than a frequent dribble because it wets the whole of the soil and the plants get some good-of it. This they do not get when only the surface is moistened. (7) Water in the as the temperature rises, never as it tends to cool the plants and the atmosphere, hence may work damage when the temperature is declining. (8) Water early enough in the day so the foliage will be dry before evening. (9) Water little if any in dull and muggy weather. (10) Use soil well sup plied with sand and vegetable matter. These materials tend to counteract the puddling effect epf watering soils in which there is clay. (11) Ventilate both to lower temperature and reduce moisture in the greenhouse. This is more im portant than the admission of “fresho air, enough of which enters through cracks and doors, (12) When admitting air avoid drafts especially of cold air. Hence have ventilators as far from the plants as possible and allow the air to enter through numerous small open ings rather than fewer large ones. (13) Pro. vide shade in summer to reduce both heat and light. This will tend to prevent spindly de. velopment of the plants.