How the Plant Increases Its Substance 



29 



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. 3pU& 

 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 to water. 

 The second layer is a tier of cells 

 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- 

 t e r 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 



WATER FPOM TH- SO/L 

 I 'CONTAINS A/f4#AL 



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. 



OF TMl 



