66 HISTOLOGY OF MEDICINAL PLANTS 
In star-anise seed (Plate 5, Fig. 2) the walls are irregularly 
thickened and wavy in outline. 
In stramonium seed (Plate 5, Fig. 3) the walls are very ° 
thick, wavy in outline, and striated. 
PLANT HAIRS (TRICHOMES) 
In histological work plant hairs are of great importance, as 
they offer a ready means of distinguishing and differentiating 
between plants, or parts of plants, when they occur in a broken 
or finely powdered condition. There is no other element in 
powdered drugs which is of so great a diagnostic value as the 
plant hair. The same plant will always have the same type 
of hair, the only noticeable variation being in the size. In 
microscopical drug analysis the presence of hairs is always noted, 
and in many cases the purity of the powder can be ascertained 
from the hairs. Botanists seem to have given little attention to 
the study of plant hairs. This accounts for the fact that in- 
formation concerning them is very meagre in botanical literature, 
and, as far as the author can learn, no one has attempted to 
classify them. In systematic work, plant hairs could be used 
to great advantage in separating genera and even species. 
Hairs are, of course, a factor now in systematic work. The 
lack of hairs is indicated by the term glabrous. Their presence 
is indicated by such terms as hispid, villous, etc. In certain 
cases the term indicates position of the hair as ciliate when the 
hair is marginal. When hairs influence the color of the leaf, 
such terms as cinerous and canescent are used. In all the cases 
cited no mention is made of the real nature of the hair. 
In systematic work, as in pharmacognosy, we must work 
with dried material, and it is only those hairs which retain 
their form under such conditions which are of classification 
value. 
Hairs are the most common outgrowths of the epidermal 
cells. They are classified as glandular or non-glandular, accord- 
ing to their structure and function. The glandular hairs will 
be considered under synthetic tissue. 
Each group is again subdivided into a number of secondary 
groups, depending upon the number of cells present, their form, 
