OF CEYLON AND INDIA. 
415 
There is of course no direct evidence, other than that 
afforded by a general comparison, for the view that the 
small size and little complexity of the secondary shoots in 
the genera named is actually due to a reduction, but it 
would seem not unlikely that such has been the case. 
A very noteworthy general feature in many of the thalli 
is their exogenous branching, and a brief consideration of 
the various cases may help to throw some light on the 
meaning of the difference between the two methods of 
origin of new organs, endogeny and exogeny. In the case 
of an ordinary subterranean root, it is easy to see that 
endogeny is advantageous to the lateral roots, and perhaps 
also necessary. The extreme tip of the root is forcing its 
way through the soil, and it would be difficult for either 
lateral roots or hairs to form on it without injury. By the 
time that the lateral roots do form, the surface cells have 
become full grown and comparatively thick-walled, and it is 
perhaps almost impossible for them to become again fully 
meristematic in such a way as to give rise to a new organ. 
It is therefore possible that there is an element of necessity 
in the endogeny of the branch root, as well as of advantage. 
This view is confirmed by the behaviour of adventitious 
roots formed upon the stem or upon old parts of the root, 
or even upon leaves ; as a rule they are also endogenous, 
though it often seems to be of no advantage to them that 
they should be so. Their origin is presumably determined 
by heredity or by the physical conditions of the surface 
tissues. Thus perhaps on the whole it comes about that 
endogeny is fixed in the heredity of the higher plants as the 
customary mode of origin of roots in general. The same 
reasoning also applies to the case of shoots developed upon 
the roots, which are also almost always endogenous. 
In the Indian Podostemaceæ, we have a case of very plastic 
organisms in which, while they are apparently descended 
from plants with endogenous root formation and branching, 
the advantages of endogeny, and apparently also the 
necessity for it, have disappeared, so far at any rate as the 
