46 



PLANTS 



98. All parts of the plant are covered, at least during the 

 early stages of their development, by a superficial layer of 

 cells forming the epidermis. On parts exposed to the air the 

 outer walls of the epidermal cells are cutinized, which renders 

 them impervious and tough. These properties render the 

 epidermis well fitted to prevent desiccation of the underlying 

 tissues and to protect them from mechanical injury. 



99. On the smaller rootlets, which are always surrounded 

 by the moist soil and hence not subject to either desiccation 



Fig. 20. — The epidermis of various plants showing different degrees of cu- 

 tinization (in black). A, Leaf of Avicennia, a Xerophyte; B, the epidermis of 

 an apple (fruit) ; C, petal of Japan quince; D and E, upper and lower epidermis of 

 leaf of Hibiscus Moscheutos; F, epidermis of leaf of prickly lettuce, Lactuca 

 scariola, in the sun; G, same, in the shade. (From Stevens.) 



or mechanical injury, the epidermis is not cutinized, conse- 

 quently it offers no obstacle to the transfusion of water and in 

 fact is here specially modified for the function of absorption 

 through the medium of the root hairs, which are only expansions 

 of some of the epidermal cells. 



100. Structures called hairs are also developed on aerial 

 parts of the plant. These assume as endless variety of forms 

 and serve various functions. Some are glandular, others are 

 organs for water absorption, and still others serve a variety of 



