238 PLANT ANATOMY. 
gradually become destroyed. In this manner the hollow stems 
(Graminee, Umbelliferee, Dulcamara) and roots (Rhiz. Grami- 
nis) are produced. Some of the air-cavities in many reots and 
leaves (aquatic grasses) may also be so produced; at least shreds 
of the cell-membrane are often found on their margins. Lastly, 
there may also be mentioned the cavities which are formed in 
many drugs by the ultimate laceration either of lines of cells of 
the medullary rays (Radix Bardane seu Lappe, Carline) or of 
Fie. 154.—Surface view of the epidermis of a leaf of Mentha piperita; ep, undulated 
epidermal cells; sp, stoma; 4d, oil glands (seen from above) with a group of crystals of 
menthol m (Tschirch), Compare also Fig. 129. 
the bark (especially of the bark-rays, Rad. Arnice, Fig. 182 6, 
R. Levistici, R. Pimpinelle). 
It is, however, more especially the lower side of leaves which 
is abundantly aérated (transpirating tissue, Figs. 127, 129). 
Here are also chiefly found the canals of exit, the stomata, 
which are mostly located at the level of. the epidermis or are 
only slightly depressed below the latter (Figs. 129 sp, 155), and 
