PLANT STUDIES 



PART n, THE WORK OF FOLIAGE LEAVES 



The little birds sang as if it were 



The one day of summer in all the year, 



And the very leaves seemed to sing on the trees. 



LOWELL, "The Vision of Sir Launfal." 



In the middle of a hot, summer day, 

 when we sit down under a shady tree 

 to rest and eat our lunch, we feel es 

 pecially grateful to nature for provid 

 ing us with such a cool resting place; 

 tut, after all, we must not forget that 

 Nature, though willing to share her 

 bounties is first considering her own 

 children. The leaves that protect us 

 from the burning sun have very import 

 ant work to do for the tree; so import 

 ant, indeed, that without such work be 

 ing done, the tree could not live. Before 

 we consider this work in detail, let us 

 examine a single leaf carefully. 



A leaf usually consists of the flat, 

 green, expanse which we call the blade ; 

 of the petiole, or little stem, by which it 

 is attached to the twig or plant ; and very 

 often of two small leaflets, found at the 

 base of the petiole, called stipules. You 

 will notice, as you have often done be 

 fore, the ribs and veins which trace the 

 leaf. In the lily, these run parallel, 

 while in the maple they form a network 

 over the leaf which is said to be netted 

 veined. These ribs and veins are 

 much woodier than the rest of the blade, 

 which is soft and pulpy in nature, and so 

 they give firmness to the leaf. They are 

 composed of hollow, woody, fibres, and 

 not only give support to the leaves, but 

 act as canals to carry the water and min 

 eral substances which are needed by the 

 leaf in the process of food manufacture. 



This brings us to the most important 

 work of leaves food manufacture; for 

 leaves are the workshop of the plant, and 

 within their cells the raw material gath 

 ered from the soil and air is made into 

 material that can be assimilated by the 

 plant. If you find a very thick leaf you 

 can strip off from it a thin layer, the 

 epidermis, or outer covering of the leaf. 

 Looked at through a compound micro 

 scope, there will be disclosed many little 

 openings, each of which is protected by 

 guard cells which change their shape 

 from time to time so as to increase or 

 decrease the size of the opening. These 



openings are called stomata; mouths, 

 literally ; their work is not fully known, 

 though they are believed to be useful in 

 both transpiration and respiration. At 

 any rate, they permit direct interchange 

 of air and light between the outside at 

 mosphere and the inner cells of the leaf. 

 This inner layer of the leaf, known as 

 the mesophyll, is made up of cells that 

 contain small green bodies, chlorophyll 

 grains, that give the green color to 

 plants. In the work of food manufac 

 ture, it has been found that carbon diox 

 ide is taken up by the cells, the carbon 

 used, and the oxygen given back to the 

 air. For this reason growing plants 

 about us contribute to our good health, 

 since carbon dioxide is a poison rejected 

 by us when we breathe, while we need 

 plenty of oxygen. After the food is 

 made by the leaves, they gradually send 

 it out through the veins to the main 

 branch of the plant, whence it is con 

 veyed all over the structure. 



A second work performed by the 

 leaves is called transpiration ; by which 

 is meant the elimination of the moisture 

 not needed by the plant. One could call 

 the process evaporation except that it is 

 controlled by the living organism; in 

 dry countries the moisture is conserved ; 

 on the other hand, where the plant ob 

 tains a great deal of water, much of it is 

 given off. Transpiration is carried on 

 by each part of the plant, but the main 

 work is done by the leaves. The truth 

 of the statement that leaves transpire is 

 easily tested by placing a glass over a 

 small growing plant; moisture will soon 

 gather on the sides. If one can note 

 the moisture given off by a small plant, 

 consider the quantity of water lost each 

 day by a forest. One can readily see 

 that there would be sufficient to make 

 an appreciable difference in the climate 

 of the surrounding country. Even a 

 meadow or a strip of lawn, a single tree, 

 or a few plants allay the heat and make 

 more habitable the place where they are. 



The third work of the leaves is res- 



