172 Leaves and their Work 



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

 few pores, and skin so thick and leathery that evapora- 

 tion 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 colour ; 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 is merely concealed from view, hidden by other 

 colouring matter. 



But some plants never have any leaf-green under any 

 circumstances, and, therefore, being non- manufacturers, 

 they have to live by the labour of others. Among 

 these are the fungi, which grow and feed entirely upon 

 organic matter, animal or vegetable, and are inde- 

 pendent of the light. Mushrooms, for instance, may 

 be grown in cellars ; toadstools spring up in the night ; 

 for their food of all kinds mineral food, nitrogenous 



