326 



CASSELL'S P0P13LAE GAEDENIXG. 



te secured in order to render our artificial climates 

 natural. A man's reason, indeed, must tell Mm that 

 a plant condemned to pass its life in a still atmo- 

 sphere is like nothing so much as a criminal set fast 

 in an everlasting pillory. In order to secure motion 

 in the vegetable kingdom, currents of air are made 

 to do the work of the muscles, limbs, and volition of 

 animals. It is not at all improbable that, in addition 

 to the mechanical effects of motion in the propulsion 

 of the sap, it may be important that the stratum of 

 air in contact with the leaves of plants should be 

 incessantly shifted, in order to enable them to pro- 

 cure an adequate supply of food ; for we find that 

 water in motion feeds them better than that which 

 is stagnant. Leaves are continually abstracting 

 from the air the very minute quantity of carbonic 



gather that the aeration of modem hot-houses by 

 night as well as by day is now of paramount impor- 

 tance, otherwise it can only be serviceable to the 

 plants so far as pei-spiration is concerned, which 

 goes on principally during the day ; while feeding, 

 the other great operation, goes on during the night. 

 Formerly, under the old system, the chink of night 

 air, now so well understood, was of less importance, 

 as plants prospered fairly weU where the squares of 

 glass were small, because there were more openings 

 for small currents of air from within and without. 

 Modem glazing has swept all these minute safety- 

 valves away, and practical men, who have devoted 

 their attention to ventilation as a science, have sub- 

 stituted methods by which the temperature can be 

 regulated to a nicety, and the plants can be treated 



Pig. 74. 



Fig. 75. 



acid which it contains. When the air moves quickly 

 over their surface, fresh supplies of that food are 

 incessantly presented to it, and the operation of 

 abstraction may be facilitated, while, on the con- 

 trary, if the air is stagnant the absorption of car- 

 bonic acid may be much slower. 



" Perspiration is another function which must be 

 maintained in healthy action. The quantity of 

 water that flies off from the surface of a plant will, 

 eieteris paribus, be determined by the rapidity of the 

 motion of the air passing over its surface. In an 

 absolutely stiU air perspiration wiU be reduced to its 

 minimum, and it will increase, within certain limits, 

 in proportion to the quickness wjth which the air 

 sweeps over it." "If the motion of air is thus 

 favourable to the two great operations of f eediug and 

 perspiration, we shall find tha,t it is equally needed 

 day and night ; for perspiration goes on principally 

 during" da,ylight, and feeding in the hours of dark- 

 ness. A good system of aeration ««««< then he con- 

 stantly in action. How to secure that, is the great 

 horticultural problem which now remains to be 

 solved." 



From the preceding remarks, which wiU stand the 

 test of all time, and which every zealous young 

 gardener should have printed in letters of gold, we 



to a continuous supply of warm fresh air, without 

 which they would languish and die. 



Size of Ventilators. — Before proceeding to 

 describe some of the best modes of admitting fresh 

 air into hot-houses, it may be well to consider the 

 sizQ and position of the inlets and outlets needful to 

 provide for the escape of vitiated and the admission 

 of fresh air to take its place. Mr. Hood, in his 

 remarks on VentUatiou as applied to Rooms, says 

 he recommends "that the aggregate area of the 

 openings for the admission of fresh air should be 

 larger than the total area of the openings for the 

 efiSux of the vitiated air. By this arrangement the 

 velocity of the entering euiTent is reduced, and 

 draughts are avoided. It is also expedient to divide 

 the entering current as much' as possible." " Pro- 

 vided the aggregate openings for the admission of 

 cold air be not less in size than those for the emission 

 of the heated air, the quantity of air which enters 

 depends less upon the size or number of the open- 

 ings which admit the fresh air than upon the size of 

 those by which the vitiated air is carried off. This 

 arises from the room being always absolutely f uU of 

 air ; no more air can enter untU a portion of that 

 already in the room be removed." Kow, althougli 



