Leaves and Their Work 



143 



in rain, damp, and darkness; and it is when they open 

 most widely that the manufacture of food goes on most 

 briskly. For it is then that most carbon is separated, and 

 most food is pumped up from the roots, as that is the 

 time when the plant transpires most, and in this way both 

 kinds of food are received together. When there is much 

 transpiration, and water containing dissolved food is 

 pumped up rapidly, then also much carbon is received, 

 and vice versa. 



Such plants as the cactus, which have no leaves, very 

 few pores, and skin so thick and leathery that evaporation 

 is prevented, transpire very little, and grow, in consequence, 

 very slowly. The tall torch-thistle cactus of Mexico is 

 said to take some hundreds of years in attaining its full 

 size; whereas the thin-leaved gourd of the East is noted 

 for its very rapid growth. 



How the food from the soil and the food from the air 

 are combined, and distributed from the leaves to all parts 

 of the plant, is unknown; but from them each part does 

 receive its due share of nourishment, one more of this 

 sort, one more of that. 



But without the leaves no food can be prepared, except 

 where the stems take their place; and without light the 

 leaves have no power to act. Hence the plant's whole 

 life depends upon the sun. 



In the autumn, when the plant has finished growing, 

 no more leaf-green is formed, and' the leaves begin to 

 change color; for instead of manufacturing food, they 

 are giving up their own stock to feed the young fruit. 



Some plants, such as lichens, copper-beeches, and 

 others, might be supposed to possess no leaf-green, 

 because it is not visible; but they have it all the same; it 



