176 PHARMACEUTICAL BOTANY 
they are cut and upon whether cut from the daughter or parent 
tuberous root. In general, the commercial root shows the fol- 
lowing internal structure: An outer layer consisting either of an 
epidermis or, in older parts, of one or more rows of subertzed cells 
with blackish-brown walls, beneath which in younger portions of 
the root occur a primary cortex of starch containing parenchyma in 
which occur scattered stone cells of numerous shapes and a modified 
endodermis, all or a portion of these regions being suberized in older 
parts, forming a brown tissue called metaderm or entirely exfoliated; 
an inner bark of numerous rows of starch parenchyma and scattered 
islets of sieve tissue. Stone cells may occur in this zone directly 
under the endodermis. Next, a five- to eight-angled cambium, 
in each angle of which is a 2- to 4-rayed collateral vascular 
bundle. In some sections of Aconite roots smaller bundles may 
be found between the angles and along the cambium line. In 
the center is found a broad, five- to eight-rayed pzth composed of 
large parenchyma cells. The parenchyma cells of the cortical 
regions and pith contain single or two- to five-compound starch 
grains. 
Root Brancutnc.—The branches of roots are formed by the 
pericycle. The cells of this region adjacent to the protoxylem 
undergo division by laying down tangential walls forming a mass 
of meristem. This meristem develops a root tip which pushes its 
way out laterally through the tissues of the primary root (Fig. 108). 
In its passage outward, enzymes are secreted by its cells which 
dissolve the tissues in its path, the root growing into the cavity 
thereby formed. The first branches ofa root are termed secondary 
roots. ‘These may give rise to branch roots and those to further 
branches by the same method. 
Root TuBERCLES 
The roots of plants of the Bean and Bayberry families as well 
as some species of the Birthwort family and of the Alders, Cycads, 
Spruces, etc. are characterized by the appearance upon them of 
nodule-like swellings called root tubercles. 
Lecuminous TuBERCLES.—In the case of the members of the 
Bean family (Leguminose), the causative factor is a species of 
bacteria named Pseudomonas radicicola. This is a motile rod- 
