THE EPIDERMIS 131 



Pteridophyta. It must be confessed that the actual organization 

 of the sporangium is a better indication of its morphological sig- 

 nificance than is any relation in origin to any particular tissue 

 system or any combination of tissue systems. Taking the vascular 

 plants as a whole, it seems evident that the sporangium probably 

 antedated the clear differentiation of the tissue systems and is 

 accordingly best regarded as an organ sui generis and as belonging 

 consequently to a distinct anatomical category. It will be shown 

 in what follows that there is good evidence in certain instances 

 that tissue systems other than the epidermis enter into the organiza- 

 tion of the sporangial structures of the seed plants. 



In conclusion it may be stated that the epidermal structures 

 show an exceptional degree of plasticity to the molding influences of 

 environment, a condition which makes them of less value from the 

 phylogenetic standpoint. This highly variable system constitutes 

 the boundary between the organism and its surroundings, whether 

 fluid, gaseous, or solid. It consists normally of a single layer of 

 cells which in certain drought-resisting plants is multiplied to con- 

 stitute what is ordinarily called water tissue. The epidermis is 

 perforated by pores known as stomata and produces outgrowths 

 known as trichomes or hairs. The former structures consist of a 

 pair of cells usually highly responsive to the combined presence of 

 light and carbon dioxide and surrounding an occlusible pore. True 

 stomata are confined to the spore-producing generation of the 

 higher plants (from the Bryophyta upward). In spite of erroneous 

 statements to the contrary, in the case of certain badly preserved 

 fossil plants there are never more than two guard cells related 

 to a stomatic opening. Assertions contradicting this are based 

 on the interpretation of accessory cells as guard cells, where the 

 former overlie the elements guarding the stomatic pore. Neither 

 the shape of the epidermal cells nor the organization of the stomata 

 nor the structure of the hairs or trichomes can be regarded as having 

 a very high importance from the standpoint of the anatomical 

 identification of extinct plants in the absence of the confirmatory 

 evidence presented by external form and internal anatomy. 



