416 WILLIS : MORPHOLOGY OP THE PODOSTEMACEÆ 
lateral rather than vertical branching is concerned. Verti- 
cal organs will be more subject to scour, and perhaps to 
them it may be advantageous to be endogenously formed. 
We have then left, the powerful influence of heredity, and 
the time of appearance of the new organs, whether at the 
earliest stages near the apex, or later. In the case of the 
secondary shoots upon the roots, even when they are formed 
near to the tip, endogeny is the rule without exception. 
That there is perhaps some very strong reason for this may 
be inferred also from the fact that even in Lawia, with its 
shoot thallus, the secondary shoots are formed endogenously, 
on the mature parts of the thallus, though perhaps their 
late origin, when the surface cells are hard and full of 
silica, may be enough to account for this. Beijerinck^ has 
shown that in Aristolochia Clematitis the secondary shoots 
show endogenous development when formed on old parts of 
the root, exogenous when on young ones. 
In the branching of the more modified root-thalli, on 
the other hand, endogeny is distinctly rare ; it occurs in 
Farmeria, but here the branches are formed very late in the 
development. It also occurs in Dicræa when branches are 
formed later in life on old parts of the root, but never at the 
apex, unless in case of rejuvenescence from an injured tip. 
In Tristicha the branching is endogenous, but it occurs at 
some distance from the tip. In. most of these forms, how- 
ever, the branching occurs quite close to the tip of the 
thallus, where the tissue is still fully meristematic, and is 
exogenous. 
It is evident, then, that like other morphological features, 
endogeny and exogeny, fixed though they are so long as 
there is no serious change in function or circumstances, 
may be modified in accordance with changes in these 
conditions. Perhaps we may regard as one of the determin- 
ing causes in the first differentiation of these two modes of 
* Beob. u. Betr. ii. Wnrzelknospen u. Nebenwurzeln. Verb. k. Akad., 
Amsterdam, 1886, d. 25,- 
