92 THE ELEMENTS OF VEGETABLE HISTOLOGY 



on the exposed surfaces of epidermal cells may be 

 due to striations, beading or presence of plant hairs. 



. Thickened Epidermal Tissues. — The epidermal tis- 

 sue covering seeds and certain classes of fruits is in 

 the form of one or more layers of thick-walled cells, 

 constituting the testa or outer coat of seeds and the 

 exocarp of fruits (Plate 34). Stomata are never 

 present in seed epidermis but are occasionally present 

 in fruit epidermis. Protection of the embryo and 

 of the nourishing materials in the seed is the chief 

 function of the testa. The seed epidermis is neces- 

 sarily thicker and stronger than the membranous 

 epidermal tissues, because the seed must retain its 

 vitality for long periods even under the most adverse 

 conditions. The epidermal cells of seeds and fruits 

 usually contain pigment materials and therefore 

 appear colored. The subepidermal layers may secrete 

 mucilaginous substances through modifications of the 

 cell walls. The swelling which occurs when the mucil- 

 aginous substance comes into contact with water 

 tends to rupture the tough epidermis, thus making 

 possible the egress of the embryo. Waxy secretions 

 are occasionally present on the surface of the seed. 



PLANT HAIRS 



Plant hairs or trichomes are prolongations or out- 

 growths of the epidermal cells. In certain instances 

 the trichome is merely an expansion of the exposed 

 surface of the epidermal cell, while in others, cell 

 division has occurred and the hair consists of several 

 well-defined cells. Trichomes may occur on the epi- 

 dermal surfaces of leaves, green stems, floral organs, 



