48 GROUPS OF TISSUES, OR TISSUE SYSTEMS 



appendages are well developed. It is worthy of note, 

 however, that those plants of these groups that have 

 reassumed the aquatic habit have their epidermis scarcely 

 distinguishable from the rest of the tissues. The roots 

 of most plants, being usually in moist soil, have their 

 epidermis not very strongly differentiated. 



65. The Epidermis. In most cases the epidermis 

 consists of a single outside layer of cells which surrounds 

 the whole plant in an almost uninterrupted sheet. It 

 frequently originates from an apical cell or group of cells 



distinct from those producing the 

 rest of the tissues, or is differen- 

 tiated from the latter near to the 

 growing point. Mostly the epider- 

 mal cells may be considered as a 

 special kind of parenchyma tissue 

 with a protective function. In 

 many plants, however, especially 

 those of hot, dry climates, the cells 

 soon become thickened and more 

 or less sclerenchymatous. Usually 

 they remain alive, but in the forms where they have 

 been changed to sclerenchyma the contents commonly 

 die. In most cases, epidermal cells show no well de- 

 veloped chloroplasts although the cell sap may be brightly 

 colored. 



66. In shape, the epidermal cells are usually more or 

 less flattened parallel to the surface of the plant. If 

 the growth of the organ is nearly equal in length and 

 width, the epidermal cells seen from the outside will be 

 nearly isodiametric, but if the longitudinal growth has 

 been markedly greater than the transverse growth, the 

 epidermal cells will usually be elongated. Frequently 

 the cells are very irregular in outline. Except for the 



Fig. 



20. — Epidermis, with 

 stomata. 



