556 Bower .—On Medullation in the Pteridophyta. 
come of general morphological experience. At once the words ‘ in all cases * 
and ‘must’ in his latest statement will attract the critical attention of 
morphologists. They can quote abundant cases of parallel development 
and homoplasy as shown in the external form of plants. There is no 
reason to hold that the evolution of internal structure differed in such 
respects fundamentally from that of form. We learn from comparative 
study that plants are opportunists in their internal as well as in their 
external evolution, developing along lines of least resistance, and domi¬ 
nated neither by logic nor by rule. Consequently, in the study of their 
tissues, strong and consistent antitheses are not to be expected, and so 
we shall, from prior experience, be prepared for divergences of detail and 
of method in the formation of such a tissue as pith in the several phy- 
letic lines. Generalizations in terms of ‘ must in all cases ’ require to 
be based upon exhaustive knowledge of detail in order to meet this 
general probability, and will have to be reconsidered in the light of even 
a single discordant fact. Thus the mode of statement adopted by Pro¬ 
fessor Jeffrey, as a dogma rather than as a working hypothesis, at once 
challenges prior morphological experience. 
We may, however, pass from this general objection to the statement 
itself. It must first be premised that there are those who hold that the 
limits of the stele are not constant ; in fact that the endodermis is not 
an immutable phyletic barrier between the stele and the cortex. I have 
no wish to prejudge this question, and myself hold it as open to de¬ 
monstration. But for the moment we may here proceed on the presump¬ 
tion that the limits of the stele have remained constant. For us the 
endodermis will be a convenient indicator of tissue-locality, and we may 
adopt * without prejudice ’ the consequent terminology. There are, in 
that case, three possible sources of the pith, as it is seen in Vascular 
Plants at large. 
(i) It may be always extrastelar in origin. Subject to the saving 
clause already quoted, Professor Jeffrey holds that this is the constant 
and only source of the pith in all Vascular Plants. 
(ii) It may be always intrastelar in origin. With regard to the 
upholders of this view we learn from Professor Jeffrey, 1 that * the English 
Anatomists . . . regard the pith as in all cases a specially differentiated por¬ 
tion of the fibro-vascular tissue itself’; but he adds that ‘of late the 
extremity of the English view appears to be modified somewhat by the 
admission that in certain instances the pith may be derived from outside 
the stele’. Professor Jeffrey neither names these ‘English Anatomists’ 
who hold the extreme intrastelar view nor quotes from their works. As I 
do not share this view, it may be dismissed without further words from the 
present discussion. 
1 1. c., p. 403. 
