FUNCTIONS OF LEAVES. 71 



veins is arranged in two layers firmly united, the superior part arising 

 from near the pith and the inferior from the liber. The former con- 

 nects the leaf and wood and the latter the leaf and bark. Thus as 

 the sap ascends through the alburnum and wood and then descends 

 through the liber, the upper system of veins communicates with the 

 ascending and the lower with the descending current. 



The skin is filled with small cavities, at first charged with fluid and 

 lastly with air. It is hard and thick in hot climates and thin in moist 

 and shady places, but it varies according to species. Some leaves 

 contain more parenchyma, or cellular texture, than others, and are 

 accordingly more juicy and pulpy. The cells vary greatly in size ; 

 in some leaves they may be seen with the naked eye. They secrete 

 fluids like glands and communicate with the vascular system. 



The leaf breathes and perspires through innumerable small pores, or 

 stomates, which are larger or smaller according to the above circum- 

 stances, being large and abundant in moist shady places, so as to ob- 

 tain liquid aliment, and also for exhaling and inhaling gases. Leaves 

 present to the air a more extended surface than all other parts of the 

 plant, and are thus of the greatest importance in deriving nourish- 

 ment from it, and giving off gases which are useless or prejudicial to 

 the plant. 



The natural habits of the plant may be known by a microscopic ex- 

 amination of the leaf-skin, or epidermis and its stomates. Those with 

 a thick skin and few stomates will naturally inhabit dry places where 

 liquid food is scarce, while those with a thin skin and numerous and 

 large stomates belong to humid climates and damp soils : intermediate 

 structures are governed accordingly. But the relative size of stomates 

 is generally a more important mark than their number. The number 

 of these in an inch of skin on some leaves, aloes for example, is 45,- 

 000, and in some others 70,000. Still, the size of those most numer- 

 ous may not be equal to those of a less number. 



The functions of leaves thus show them to be the lungs and stomach 

 of plants, having a system of veins, etc. Their skin is an extension 

 of the skin of the stem, which, in some plants, performs the functions 

 of leaves when leaves are wanting. Respiration, perspiration and 

 digestion, as performed by the leaves are not less essential to the life 

 and growth of plants than are the same functions to men and lower 

 animals. The primary agent in the performance of these functions is 

 solar light. This, in striking the leaf, causes a decomposition of 

 carbonic acid and the extrication of nitrogen, with an insensible per- 

 spiration. 



Foliation is the manner in which leaves and flowers are enclosed 

 oy the scales of the bud, which is so varied that the families of plants 

 may be distinguished by it. Plants which are destitute of leaves are 

 called aphyllous ; i. e., wanting a leaf. To the various characters of 



