24 ELEMENTS OF UOTANY. 



We have already seen thah leaves arc eonipoHed of cellular and woody 

 tifisi:.., and have considered the latter in its raniilications whi(;h make up 

 the leaf-skeleton. We will now briefly examine the cellular tissue. 



Unlike the cellular tissue of the stem, 

 this is a gretiii pulp closely reseniLling the 

 greou layer of the Lark. It is made u\i of 

 cells somt^Lat loosely arranged, with oi)en 

 spaces or air-passages between them (Fig. 

 35). These cells ovvo their green color to 

 minute grains of a peculiar green coloring 

 matter, termed chlorophyll, which they 

 contain. Externally th(> entin; leaf is covered 

 with a thin, transparent membrane, termed 

 V^"^'^''"^— epidc mis (Fig. 3G) ; this is perforated 

 Pig. .-jr-Riirf.ioo of a loaf, (.howhiK Kt,. y^^.{^]^ numerous openings, termed stomata 



nmta (bi-iiilliinfri.i>rL's). Magnitiud. i • , 



(iMg .W), winch iK'rmit the external an- to 

 have free access to the interccUuhu" air-passages. The stomata arc nuich 

 more numerous on the under than the upper side of the leaf, and here 

 also the air-passages ax'e most abundant. 



FUNCTIONS OF THE LEAVES. 



Leaves have often been compared with the lungs of animals, since it is 

 their office to aerate the vital fluids of plants. The nourishment collected 

 by the roots is transmitted through the stem to the leaves, and here, ex- 

 posed to contact with the «'L', it becomes elaborated and fitted for the 

 plant's further use. Through the multitude of stomata, or breathing-pores, 

 the air has free access to the interior of the leaf, where the cells take from 

 it carbonic acid and yield up their superfluous moisture, or absorb oxygen 

 and water as may be required. Li sunlight leaves absorb carbonic acid 

 and give out oxygen ; in darkness the process is reversed and carbonic 

 acid is exhaled. But as plants are much more active in daylight than in 

 darkness, the amount of carbonic acid taken from the atmosphcx is many 

 times groater than that which is exhaled ; and as nearly all the carl)onic 

 acid absorbed is decomposed, the carbon alone being retained while the 

 o\ygen is returned to the air, it at once becomes evident that plants are 

 continually purifying the air which animals breathe. Animals, on the other 

 hand, are as constantly renewing the supply of carbonic acid in the air, 

 and thus better fitting it for the sustenance of plants, so that there is an 

 intimate interdejiendence of veget!d)ic and aniuial life. Both probably 

 liad their advent u]ion earth at the same time, and progr(>ssed upward 

 from the lowest to the highest forms, side by side, with equal stei)a. 



So far the leaves anj analogoiis to the lUngs of animals, but their func- 

 tions do not cease with the mere absorption of cai'bonic acid and '.he ex- 



