How the Plant Increases Its Substance 



29 



to water. 



layer is a tier of cells 



ITYLEDOMS 



to be composed of a great number of cells. The surface 



layer forms a skin, or " epidermis,", which keeps the 



cells within from 



drying. (Fig. 



20.) The epi- 



dermis is in 



two layers. The 



outer, or cutin 



layer, is only a thin 



membrane which, while 



transparent, to allow the 



light to reach the inner 



tissues of the leaf, is 



impervious 



The second 



which support the cutin layer. This 



epidermis is very efficient in keeping 



the water in the leaf. On the lower 



side of the leaf, and on both sides 



of some leaves, 



there are many 



small openings, 



to let the car- 



bon dioxid en- 



ter and the 



excess of oxy- 



gen pass out 



when the plant is making food. (Fig. 21.) Some water 



escapes through these openings, or stomata (singular, 



stoma)-, but at night, when the food-making processes 



are not going on, these stomata close up, so that 



much less water escapes. 



47a. To get an idea of how well the epidermis protects the 



Fig. 21. How the young plant gets its food. In the early 

 stages it is nourished from the store of food in the 

 cotyledons. When the green leaves unfold to the 

 light they absorb the energy of the sunlight and 

 cause the water to combine with the carbon dioxid 

 of the air to form starches and other foods. 



