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Birbal Sahni. 
hollow cylinder lined internally as well as externally by phloem, 
pericycle and endodermis, while the centre is occupied by a patch 
of ground-tissue which in this region is sclerenchymatous. This 
Fig. 1. Nephrolepis cor difolia. A .—Slightly diagrammatic representation 
of the vascular system of the tuber and its relation with that of the stolon. 
The broken line indicates the outermost limit of the cortex in both tuber and 
stolon. R, roots; their points of origin have no relation to the gaps in the 
reticulate stele of the tuber. B .—Transverse section of the tuber through its 
broadest part. None of the gaps have any relation to leaf-traces , whether rudimen¬ 
tary or other. 
state of affairs does not, however, long persist, for, as the cross- 
section of the tuber increases, the stele also expands further and 
the xylem ring (Fig. 2, A) which has in places already become 
attenuated to only one or two layers of tracheides, becomes 
disintegrated into usually three or four arcs separated by gaps, with 
the result that the internal phloem becomes continuous with the 
external at the edges of the gaps. The arcs of xylem become 
further and further removed from the centre and from each other. 
Through the widening gaps first the internal pericycle, then the 
internal endodermis, and finally the central ground-tissue become 
continuous with the corresponding external tissues. The “ pith ’» 
has meanwhile lost its thick-walled character and can no longer be 
differentiated from the cortex. As we pass distally from the base 
of the tuber, the arcs of xylem divide repeatedly by constriction 
and give rise to a considerable number of strands which anastomose 
with each other, forming an irregular network lying parallel to the 
