CHAPTER DC 

 THE EPIDERMIS 



In vascular plants the epidermis is ordinarily a single layer of 

 cells, and this fact, together with the external position of the 

 epidermis, makes the latter extremely easy to distinguish. The 

 organization of the epidermis is often strongly influenced by condi- 

 tions of environment, and as a consequence the integumentary 

 structures have a value from the evolutionary standpoint which can 

 be very easily overestimated. On account of its relatively slight 

 phylogenetic interest the epidermal tissue will receive small atten- 

 tion in the present connection. 



In the case of the organs of plants normally exposed to air and 

 not to water or earth we find the integumentary structures con- 

 sisting of a well-marked layer of cells, which ordinarily remains 

 single and is characterized by the presence of stomata or pores. 

 The cells of the epidermis in lower forms often contain chloro- 

 plastids, while in higher forms chlorophyll-containing plastids are 

 usually found only in water plants or in those inhabiting shade. 

 Plastids or masses of organized protoplasm in the epidermal cells of 

 the higher plants are not indeed rare, but usually when found in the 

 superficial layer they lack color and consequently are included 

 in the category of leucoplastids. The chloroplastids of the epi- 

 dermal structures of Pteridophyta may possibly indicate the primi- 

 tive presence of these bodies in the integumentary tissues of the 

 higher plants, but they are perhaps equally susceptible of inter- 

 pretation as a response to a shaded or damp habitat. The presence 

 or absence of chloroplastids in the epidermal elements cannot con- 

 sequently receive a very high valuation from the standpoint of 

 determination of the relative phylogenetic position of groups of 

 vascular plants. The situation is particularly obscure on account 

 of the lack of bearing of fossil evidence on the question, both 

 because plastids are rarely preserved in the cells of extinct plants 

 and because, even if they were, it would be impossible to distin- 



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