78 
Birbal Sahni. 
could be traced. Some of the tubers were flattened, having grown 
against the wall of the pot, while others were distorted by small 
pebbles in the soil. A few were spherical. 
Theoretical. 
In the foregoing account of the vascular anatomy of the tubers 
the main feature of interest is that the different stages through 
which the stele passes from the solid, cylindrical to the net-like 
condition are closely similar to those seen in the ontogeny 
of the solenostelic and dictyostelic Ferns, in which the influence of 
leaf-traces is justifiably regarded as the dominating factor in the 
evolution of the cauline stele (Boodle, Gwynne-Vaughan, Tansley)* 
My first impulse was to conceive of the tuber as possibly homo¬ 
logous with the “lateral plant” borne normally on the aerial stolon, 
which might have undergone swelling and a great reduction in the 
leaves, so that in its modified form it may serve as an underground 
organ for vegetative reproduction and water-storage. Certainly the 
position of the tuber on the stolon is exactly the same as that of a 
lateral plant, and the similarity in the stelar condition as traced 
from the base upwards encourages one in the hypothesis. The 
meshes in the network of strands might then be interpreted as leaf- 
gaps such as are known to exist in the Cactacese, 1 in spite of the 
extreme reduction of the leaves. The hypothesis is, however, not 
to be seriously entertained in view of the fact, already noticed by 
Heinricher ( loc. cit., p. 50), that the surface of the tuber shows no 
trace of leaves, even in a most rudimentary form. 
Since then, none of the gaps can be looked upon as leaf-gaps, 
the stele in question is not entitled to be called even a perforated 
or dissected solenostele. Such a stele is not, to my knowledge, 
known to exist elsewhere among Ferns. It may be mentioned that 
in one underground stolon the strand was seen to break up into a 
few anastomosing ones arranged in a ring; these again fused up 
into a single strand at a point several millimetres from where the 
original strand divided. The portion of the stolon between these 
points showed neither any marked swelling nor any trace of leaves 
Structures which in external appearance (and apparently also 
in function) most closely approach Nephrolepis tubers are found in 
several species of Equisetum ( E . arvense, E. Telmateja, E. palustre, 
E. hiemale) in which some of the internodes of the subterranean 
rhizome undergo swelling. 1 A point worth noticing is that in these 
1 Ganong, W. F. “ Beitrage zur Kenntnis der Morphologie und Biologie 
der Cacteen.” Inaugural-Dissertation. Cited by Jeffrey, E. C., Trans. 
Canad. Inst., 1900, p. 35 of Reprint. 
