of Vascular Strands in Angiopteris evecta . 393 
essential characters corresponding to real entities, the develop- 
ment of which can be definitely traced, and which, stripped of 
subordinate details, admit of consistent comparison both with 
one another at the various stages of ontogeny, and also with 
those structures which are homologous with them in other 
plants. 
In dealing with the attempts to frame a morphological 
account of the tissues it is well to recognize at once both the 
possibilities and the limitations which necessarily attend any 
efforts made in this direction. The value, morphologically 
speaking, of a tissue appears to us to rest on a different footing 
from that of an external member such as a leaf ; and for this 
reason ; that the morphological nature of the latter is, in the 
vast majority of instances, already determined from its first 
appearance as a cellular outgrowth, and is entirely unaffected 
by the particular course of further development which it may 
undergo, and equally so by the nature of the differentiation 
which the cells and tissues composing it may exhibit. 
But in estimating the morphological nature of a tissue , or 
even of a tissue region, we are on much less assured ground. 
Our criteria only become applicable as the adult condition is 
reached, or when, at any rate, cellular differentiation has so 
far progressed that we are able to recognize the characters 
which will stamp the tissue at its maturity. 
Even the older attempts to found a morphology of the 
internal regions, which explicitly started from germinal layers, 
or what were believed to be such, left much to be desired ; 
and the efforts which have been made since that time to 
combine more or less hypothetical embryonic limits with 
observed ontogenetic differentiation can hardly be regarded 
as entirely successful. The stele, at least in the stem, admit- 
tedly corresponds for the most part with actual tissues in 
course of differentiation, but woven into it are other concep- 
tions such as phloeoterma and the like, which are of a more 
purely subjective nature. 
It is worth while dwelling for a moment on the criterion 
(endodermis) which is employed to limit the stele from the 
D d 
