76 
Birbal Salmi. 
The course of elaboration sketched above is very often slightly 
modified, inasmuch as the internal phloem may effect a junction 
with the external before any internal pericycle has made its 
appearance. (Compare Fig. 3, which is from serial sections of the 
apex of a tuber). The xylem-ring is thus converted into a horse-shoe: 
the gap of the horse-shoe is at first merely bridged over by external 
phloem, which is in turn overlain by pericycle and endodermis, but 
Fig. 3. Neplirolepis cordifolia. Transverse sections of the apex of the tuber, 
the series running from the apex downwards. The various tissues are 
indicated as in Fig. 2. Diagram A shows a stage resembling the “ Lindsaya- 
type ” of stele. For further explanation see Text. 
it subsequently becomes invaded by the latter tissues in succession. 
Disintegration of the horse-shoe gives rise to three or four xylem- 
strands arranged in a ring. This process of disintegration may 
either begin before the pericycle has advanced far into the gap, so 
that we have the xylem-strands lying embedded in a single mass of 
phloem (Fig. 3, C); or it may be delayed till after the pericycle and 
endodermis have formed a complete internal lining to the xylem. 
One cannot fail to notice the superficial but close similarity of 
some of the stages shown in Figs. 2 and 3 to those passed through 
