Evolution of Branching in the Filicales. 13 
Thus the development of a thread-like or reticulate branch, 
stele, as the case may be, from a reticulate main stele, would seem 
to depend upon whether the rudiment of the branch became dormant 
soon after its first origin or whether it was allowed for some time 
to grow side by side with the apex of the main axis before it became 
quiescent. The extremely minute size of the branch-initial in the 
former case might explain the thread-like character of the stele—a 
kind of stele which would result if the contraction of a reticulate 
stele be carried far enough. In this connexion it is rather illumin¬ 
ating to notice that even when a reticulate main stele gives off a 
similar branch stele, the latter is often rather suddenly constricted 
at the base, though still reticulate. This constriction is obviously 
due to the fact that the branch-bud was at an early stage pushed 
aside, though not at once reduced to dormancy, by the main growing 
point so that the development of the two was unequal at first. 
We have tried to analyse the biological conditions governing 
the two extreme modes of branching shown in Fig. 1, C, F. More 
difficult seems an explanation of the intermediate stages, D, E, 
where from a dictyostelic main axis arise branches which at their 
insertion are respectively at the solenostelic and the Lindsay a- 
stages. We lack the support of observed facts, and no definite 
statement can be made till further work has been carried out. At 
the same time, there seems to be nothing against our explaining 
these cases on the same principle as above, namely, that the 
formation of the type of vascular cylinder is determined by the size 
of the bud at the commencement of its activity, this size being more 
or less fixed for the given species and controlled by the biological 
conditions. 
That the conditions of nutrition exercise a far-reaching influence 
on the development of the vascular system is only an expression of 
the general principle that these conditions are all-important for the 
development of the whole plant—a principle which is the foundation 
stone of the science of experimental morphology. We shall consider 
a few examples which appear to be particularly instructive in 
connexion with the problem before us. 
In his study of Matonia pectinata Mr. Tansley 1 described some 
plants which were found in a deeply shaded rock-crevice. To quote 
his own words, “ they were, no doubt, plants of some age which 
had been unable to attain anything like their full growth owing to 
1 Tansley and Lulham, Annals of Botany, Vol. XIX, 1905, p. 490. 
