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 t>^e 

 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 Httle attention to 

 the study of plant hairs. This accounts for the fact that in- 

 formation concerning them is very meagre in botanical hterature, 

 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, 



