14 Birbal Sahni. 
want of soil and light. Their vascular system is of interest, since it 
leads up from that of the .... young plants to that of the typical 
adults.” In some plants of Helniinthostachys zeylanica Professor 
Lang 1 found that the rhizome, which had already attained the adult 
type of structure (with tubular xylem), subsequently reverted to the 
juvenile (protostelic) condition by passing through a series of changes 
involving a diminution in size of the whole rhizome. Professor Lang 
legitimately explains the phenomenon as due to growth under less 
favourable conditions of nutrition. It is of interest to find the same 
view expressed by Professor Bower in his paper on the origin of 
medullation in the Pteridophta. 2 He remarks that in the Ophio- 
glossacese the young plant may at first either have a solid xylem 
cylinder, or there may be a small pith from the very beginning ; 
and he relates the latter condition to a more efficient nutrition of 
the young plant. 
In their development such young plants are directly comparable 
with the different types of branches described above, whose steles 
at the base are medullate or non-medullate according as their 
rudiments entered at once on a vigorous life, or, on the other hand, 
became dormant while still relatively minute, and subsequently had 
to work up from a small beginning. In striking consonance with 
this idea is Professor Lang’s conclusion ( loc . cit., 1915, p. 34) that 
“ the simpler type of stele characteristic of normal young stages in 
the ontogeny of Helminthostachys is to be associated with small size 
and less efficient nutrition.” This conclusion may well be extended 
to all vascular plants developing from a small beginning. As the 
flow of food increases the axis grows in thickness, and its stele 
undergoes a corresponding dilatation ; we have here a phenomenon 
similar in essentials to that seen in the tubers of Nephrolepis 3 though 
less pronounced in degree. 
I regret that when I wrote the paper just cited I was not aware 
of the papers referred to in the footnote. 4 Of these the last- 
mentioned requires special attention. The author describes, in 
Hymenophyllum lineare , sessile or stalked tuberous bodies produced 
as side-branches from the rhizome. From the description and the 
1 Lang, Annals of Botany, 1915, p. 33. 
2 Bower, Annals of Botany, 1911, p. 550. 
8 .Sahni, B., New Phytologist, 1916, p. 72. 
4 Senn - Verh - natf - Ges. Basel. Bd. XXI, p. 115, Die Knollen von Poly- 
podium Brunei Werckle ; Christ und Giesenhagen, Flora, 1899, p. 79. 
Giesenhagen, Berichte d. deutschen hot. Ges., 1909, p. 331 and PI XV 
fig. 8. ’ 
