FORMS OF CELLS. 
47 
cells may be divided into two classes, namely, epider- 
mal cells and cork cells. 
The epidermal cells constitute the outermost layer 
of the plant. They contain the organized cell-contents, 
the plastids in some instances being wanting ; in 
some instances they also contain dissolved coloring 
principles ; and on account of the relatively large 
amount of water which they contain, they are classed 
among the important water-reservoirs of the plant. 
The walls are principally characterized by one or 
more lamellae of cutin, these uniting to form a contin- 
uous outer wall; usually cutin is present only in the 
outer wall. The cutin, is not infrequently associated 
with wax, this constituting the bloom of fruits ; less 
frequently such inorganic substances as calcium car- 
bonate, calcium oxalate and silica are present, and not 
infrequently mucilage is present, as in the walls of seeds. 
On surface view the form of these cells varies from 
nearly isodiametric to oblong; they may also be polyg- 
onal or branched. In transverse section their radial 
diameter is much the shorter. In some instances the 
inner and side walls are considerably thickened, as in 
the seeds of a number of the Solanacese. 
The epidermal layer may not only consist of a single 
layer of cells, but may have additional layers under- 
neath forming the hypodermis, as in the upper surface 
of the leaves of species of Ficus ; in some instances the 
hypodermis undergoes a mucilage modification, as in 
the leaves of buchu. 
Plant Hairs. — The epidermal cells are sometimes spe- 
cially modified centrifugally, giving rise to papillae, to 
which the velvety appearance of the petals of flowers 
is due; in other cases this modification is in the form 
of hairs. These may be unicellular or multicellular, 
and in addition the latter may be secreting or non- 
