76 COMPOUND ORGANS OF PLANTS. 



of cells. Chlorophyl is not developed in the external investing 

 layer of cells called the epidermis, nor in the woody fibre 

 through which the crude sap circulates, both the epidermis 

 and woody fibre being from this cause white and transparent ; 

 but it is formed in the superficial strata of cells immediately 

 beneath the epidermis, and gives to the leaves and young 

 shoots their green hues. 



That the chlorophyl, or green matter in plants, is produced 

 by the effect of light, is evident from the fact that it is decom- 

 posed and disappears when plants are made to grow in the 

 dark. The celery served at table is blanched or rendered 

 white by covering the stems with earth, so that the light 

 cannot gain access to them ; and for the same reason, plants 

 exposed to the full sunshine have a deeper tint than those 

 which grow in the shade. 



The epidermal system of the leaves, together with the val- 

 vular action of their pores has been already described, (page 

 22,) and we have seen how beautifully their stomata controL 

 the evaporation from their surface. But these organs have 

 other uses. They are the instruments by which the leaves 

 communicate directly with atmosphere, and by which vegetable 

 breathing or respiration is carried on. Vegetables respire as 

 well as animals, and the sap of plants which is analogous to 

 the blood of animals, must be brought into contact with the 

 atmosphere, like the blood, and be thoroughly aerated in the 

 leaves, before it can be converted into nutritive fluid. 



Bonnet was the first who observed that leaves, when plunged 

 into water and exposed to the action of sunlight, disengaged 

 gas. He also found by experiment, that the same amount of 

 gas was evolved when the leaves were immersed in water 

 which had been previously boiled, and therefore completely 



