216 



CASSELL'S POPUI-,:VE GAKDENINa. 



in itself the functions of the skin, of the kidneys, of 

 the lungs, of the digestive organs, and of the manu- 

 facturing glands. No wonder physiologists wince 

 when they see a gardener cutting off Strawberry- 

 leaves, or engaged in cutting away what has been 

 formed at such an expenditure of power, and which 

 itself contains so much latent force for use when 

 required. Nevertheless, the gardener may, from 

 his point of view, be quite correct. His require- 

 ments are not the same as those of the plant in a 

 state of nature, and long experience has taught him 

 how to make the plant subservient to his wishes. If 

 the physiologist carried out his principles he would 

 subordinate the man to the plant ; the gardener's aim 

 is just the reverse. 



In considering the work done by the leaf, then, 

 we may treat of it as : 1, an organ of perspiration ; 

 2, of breathing ; 3, of manufacture ; 4, of digestion. 



The Leaf as an Organ of Perspiration 



It needs no elaborate apparatus to demonstrate this 

 action of the leaf. Place a leaf under a bell-glass, 

 expose it to the sun, and shortly the inner surface of 

 the glass will be bedewed with moistui-e ; so it will 

 be if kept in the dark in a sufficientlj^ warm place. 

 If this simple experiment be objected to on the 

 score that, as the leaf is detached from the plant, 

 it is not a fair test, there are plenty of other means 

 of showing it not open to this objection. The 

 inside of a green-house or of a Ward's case will show 

 the fact, though in these instances the evaporation 

 is partly from the soil; but the most convincing- 

 proof of all is afforded by the withering of leaves 

 that takes place on a hot, dry, sunny day. See how 

 the Phloxes in the garden and even the Mangels 

 in the field hang their leaves imder such circum- 

 stances. The scientists can prove the fact beyond 

 dispute, and they can likewise measure its amount 

 under different circumstances very exactly, but in 

 practice we have no need for such nicety. The 

 amount given off is very large, varying according to 

 different circumstances. It is greatest when the 

 plant is simultaneously placed under the influence 

 of bright light and high temperature, and lowest 

 under the opposite circumstances. When, however, 

 the atmosphere is saturated, temperature has no 

 effect in increasing the amount of evaporation from 

 plants. Blue light and next to it red light are most 

 efficacious in promoting transpiration. Yellow light 

 is much less so, and green light is not more effective 

 from this point of view than darkness itself. 



The texture and construction of the leaves modify 

 the amount of transpiration very materially — the 

 younger and more delicate the structure, the greater 

 the amount of transpiration. The older the leaf, 

 or the thicker its skin — as, for instance, in succulent 



plants — the less is given off. Under normal cir- 

 cumstances, where the quantity of fluid given off is 

 not materially greater than can be replaced by root 

 absorption, or furnished from the stores in the plant, 

 probably no harm arises from this transpiration, 

 and the flow of water may probably facilitate some 

 of the chemical changes which go on concuiTently 

 in the leaf. But where the balance is disturbed, 

 the sufferings of the plant become evident. It may 

 be that, provided water be supplied, little or no 

 permanent harm may accrue ; but, considering the 

 amount of force or energy expended in the process, 

 and the loss of temperatm-e involved in it, it is 

 obvious that if transpiration be excessive or fre- 

 quently repeated, the plant will suffer. It is more 

 than probable, though it has not been definitely 

 proved, that much of the ill-effect arising from a 

 plant having experienced a " check " during growth 

 may be attributed to excessive or disproportionate 

 transpiration. Fortunately that is a matter easily 

 avoided by the gardener who has water-supply and 

 means of shading at command. The use of green 

 glass in conservatories, as at Kew, may in part be 

 explained by the check it imposes to too vigorous 

 leaf-action. 



How the Leaves Breathe.— In the case of the 

 lungs of animals, the inhalation of atmospheric air 

 and its passage through the lungs is effected by 

 means of powerful muscles and contractile tubes. 

 A different set of muscles promotes expiration. In 

 plants there is no such machinery available, unless 

 it be in the case of the stomata or pores {antey 

 p. 162), which contract or open according to cii'cum- 

 stances, and so regulate both the emission of water 

 and the entrance of gases. For the most part, how- 

 ever, the entrance of air into the plant is due 

 solely to the power of penetration or diffusion which 

 gases have, and which has the closest analogy with 

 the process of osmosis formerly spoken of, and by 

 virtue of which the gases pass through the skin of 

 the leaf when not too thick or obstructed by waxy 

 exudations. The breathing process in plants pre- 

 sents many analogies, some say it is identical, with 

 that of animals. In both it is a continuous process 

 going on day and night ; but while in the case of 

 the animal it is limited to the Ixmgs or gills, in plants 

 it takes place over the whole surface, where the 

 texture is favourable to the passage of gases. 



Atmospheric air when piire has the following 

 average composition : — 



Volume per Cent. 

 Oxygen .... 20-970 

 Nitrogen .... 79-000 

 Carbonic acid . . . 0*030 



100-000 



