17 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
14, A). It has been shown by Lachmann ( Thesis , Paris, 1889), and by 
Sahni (New Phyt., vol xv, p. 72, 1916) that the protostele of the stolon 
expands at the base of the distended tuber. As seen in transverse section 
it first acquires a central mass of phloem, followed successively by peri- 
cycle, endodermis, and ground-parenchyma. In fact it becomes soleno- 
stelic. As the base of the tuber expands the ring breaks up by irregular 
perforations, as it does in leafy shoots of many Leptosporangiate Ferns. 
But here there are only perforations : since no leaves are borne on the 
stolon there are naturally no foliar gaps. A network of meristeles is thus 
A, Stolon bearing a tuber, in which the protostele breaks up into a cylindrical network, 
contracting again at the apex. After Sahni. 
B, Transverse section of protostelic stolon. (x5.) 
C, Transverse section of tuber (also x 5) showing ring of meristeles each limited 
by endodermis. 
Diameter of stolon, 1*6 mm. Diameter of tuber, 11*0 mm. 
formed, each limited by a complete endodermis, and arranged in an ex- 
panded ring (fig. 14, C). At the distal end, where the tuber contracts again, 
the network narrows down through stages of condensation the reverse of 
the previous disintegration. In a given case the diameter of the stolon 
was 1*6 mm., and of its protostele -6 mm. The diameter of the tuber was 
IT cm., and of its ring of meristeles *74 cm. : that is, nearly fifteen times 
that of the original protostele. It thus appears that, while complete 
endodermal control is maintained, when the stolon of Nephrolepis dilates 
into a tuber the same features of stelar expansion appear as in the 
conically enlarging axis of many Leptosporangiate Ferns. This suggests 
that the increase in size in both cases determines the structural change, 
while the reversal of that change which follows on the apical contraction 
of the tuber strongly supports that conclusion. 
VOL. XLI. 
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