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 
many plants, however, especially 
those of hot, dry climates, the cells 
Fic. 20.—Epidermis, with .49n 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 
with a protective function. In 
