of Dioscorea prehensilis . 419 
course of development already described for the central 
vessels ; in the case of the sieve-tubes there is absolutely no 
evidence to show that such is the case. 
Before the differentiation of the xylem and phloem, adjacent 
cells fuse together at various parts of the cortex to form 
mucilage reservoirs. These are very numerous ; in one trans- 
verse section as many as 130 were counted, and they develop 
rapidly. Eventually they become filled with bundles of 
raphides, and these may possibly subserve a protective function 
for the period during which the root remains soft. In longi- 
tudinal section these sacs are seen to be long, and they are 
situated one above the other forming long strings. 
The central cylinder increases in diameter, and as it does so 
the vascular elements are differentiated. As in many roots 
of Monocotyledons the number of phloem- and xylem-groups 
is very large, thirty of each being not an uncommon number 
in the plant under discussion. 
Sieve-areas occur on the lateral walls of the larger sieve- 
tubes, and a well-marked exodermis is present at the periphery 
of the cortex. 
There is nothing further worthy of record until the general 
lignification of the vascular strand sets in. The first portions 
to thicken are the regions immediately external to the phloem- 
groups. This is illustrated in Figs. 3 and 4, from which it 
may be seen that a crescentic mass of fibres encase the 
external region of the phloem. 
In order to ascertain whether or not this peculiarity obtained 
in other plants of the Dioscoreaceae, the roots of Tamils com- 
munis and an unnamed species of Dioscorea (labelled Pehio 
Yam) were examined. In both cases, although the lignification 
may become general, it was found that the regions external 
to the phloem-groups were not remarkable in being the first 
to be so markedly lignified. 
Returning to the case of Dioscorea prehensilis , induration 
proceeds inwards towards the centre of the stele, and out- 
Asparagaceae are multinucleate ; he also finds that the development of the xylem 
is centrifugal. 
