DRACÆNA REF LEX A. 
169 
does not exist, even near the root stock. The cambial activity 
in the small part of the primary root is very irregular, and 
develops apparently in response to local irritation in the 
cortex, the cells of which had also begun to divide. 
Starting from this stage I was able by means of repeated 
peripheral measurements and microscopical examination of 
the tissues, to determine the rate at which the vascular bundles 
were added. In the plants examined,the cambium once started 
in the stem produced a radial row of four vascular bundles, 
together with the intervening parenchyma, in six months’ 
time. During twelve months’ cambial activity a radial 
row of eleven bundles had been formed. It must be remem- 
bered that since the periphery of the stem increases with 
age, the same number of vascular bundles per radial row 
represents an increased total activity in each successive 
month. 
The next point of interest was in determining the part of 
the root in which vascular cambium originated. 
Scott and Brebner state that in D, Draco and D. fragrans 
a definite relation was found between the secondary thicken- 
ing and the insertion of the branch roots. They found 
stages in which there was pericyclic thickening only, limited 
to the immediate neighbourhood of the rootlet insertion. 
One case is noted in D. Draco^ where “ no sooner had the 
secondary tissues begun to thin out in receding from a 
lateral root than they began to widen again as the next 
lateral root was approached.” It is further stated that “ the 
chief formation of secondary tissues begins at the bases of 
the rootlets and thence extends both up and down the root 
and also peripherally.” It is therefore clear that for these 
two species the cambium commences at the insertion of the 
rootlets, and the maximum thickness of the secondary zone 
is attained in this area and not at the base of the main 
adventitious root itself. 
This is quoted as being contrary to what is described for 
D, reflexa, where, according to Strasburger, the cambium 
starts and attains maximum thickness at the base of the 
