44 Commercial Gardening 



and others equally exposed to light on both surfaces. The internal struc- 

 ture in these cases is alike on both sides. The leaves of Water Lilies and 

 others which float on water have the air pores on the upper surface; while 

 leaves developed under water have neither cuticle nor air pores. The 

 tissue of a Mushroom contains neither chlorophyll nor starch. 



Work of a Leaf. The leaves and other green parts of a plant con- 

 stitute a workshop of many compartments, in which the raw food ma- 

 terials are organized into more or less simple or complex substances for 

 the building up of the various parts of the plant body. The green chloro- 

 phyll granules are the agents, under the influence of sunlight, whereby 

 these remarkable chemical changes are brought about. Starch is the first 

 visible product, and first makes its appearance in the form of small grains 

 in the chlorophyll granules. The leaf breathes by absorbing oxygen from 

 the air, and that gives it power or energy by which the other chemical 

 changes are brought about. The oxygen attacks and destroys some of the 

 material in the cells, unites with carbon to form carbon dioxide, which 

 is given off into the air as in the breathing or respiration of animals. The 

 leaf also absorbs carbon dioxide from the air, breaks it up, gives off the 

 oxygen, and the carbon unites with the elements of water brought into 

 the plant by the root. The more complex composition of the protoplasm 

 is effected in the leaf by the addition of the other necessary elements of 

 plant food absorbed by the roots (see p. 33). This process of building 

 up organized matter from the raw materials is termed assimilation, and 

 can only be carried on during daylight, though it may possibly be done 

 artificially by electric light. Plants which have no leaves, like the Cacti, 

 assimilate by means of the chlorophyll in their stems and branches. 



The effect of light and shade on the leaves of plants is not fully 

 appreciated by all gardeners. Too often we see plants crowded so much 

 together that only a very small percentage of the leaves have any 

 chance of being bathed in sunshine during the day. Fruit growers will 

 have too many trees to the acre, probably thinking that the crops will 

 be weighty in accordance with numbers. The exact reverse is really the 

 case, for the simple reason stated above that only under the influence 

 of daylight can the starch and other building-up materials be formed in 

 the cells of the leaves. The great bulk of the dry weight of any plant 

 is obtained from the atmosphere, not from the soil; hence the necessity 

 of allowing a fair amount of space between one plant or one tree and 

 another. (See pp. 108, 141.) 



Another kind of work carried on by leaves is transpiration, or the 

 passing off of watery vapour into the air. The cells bordering the inter- 

 cellular spaces when gorged with water give off some of it in the form 

 of watery vapour into the cavities communicating with the stomata. 

 During daylight, and when the air is comparatively dry, the stomata 

 open and the watery vapour passes out. The cells bordering the air 

 cavities would soon get dry and flabby, but prevent this by absorbing 

 water from cells behind them, and these in turn from cells more deeply 



