738 Gwynne- Vaughan. — Observations on the 
that region is definitely and satisfactorily delimited. It is not 
directly concerned with any question as to which particular 
segment it is in which the tangential wall appears that first 
of all delimits it. All that is required is that a definite 
delimitation should actually be possible at one point or 
another during the course of its development in a majority 
of cases sufficiently great to render the statement general. 
According to the view taken by Farmer and Hill the 
attention is concentrated upon the vascular tissue alone, and 
therefore the stele as a whole is deprived of any regional 
distinction. I quite agree with them that it is inconvenient, 
from a descriptive point of view, to draw theoretical distinc- 
tions that have no histological expression between different 
regions of the general ground-tissue. But at the same time 
I hold that it would be a great mistake to give up all 
attempts to discover the phylogenetic history, and the precise 
method of origin, in each particular case of such tissues as 
the central parenchyma, and even as the much abused en- 
dodermis. Full information upon such points is bound to be 
of considerable value, and the ability to state of two plants 
presenting the same type of structure whether they reached 
this condition by passing through the same, or through 
different series of changes would throw light upon many 
interesting problems which might otherwise remain obscure. 
For instance, in the Cyatheaceae and Polypodiaceae, it has 
been suggested above that, whether by the intrusion of the 
cortex, or by the metamorphosis of stelar tissue, the first 
appearance of the internal parenchyma must have taken place 
at the periphery of the protostele, and at points just above 
the departure of the leaf-traces. This displacement, or trans- 
formation, of the vascular tissue then advanced gradually 
inwards from these points until even the most central region 
of the stele was affected by it. The internal parenchyma, 
therefore, from the moment of its first appearance, was always 
in contact with the cortical parenchyma. 
Now several other methods of procedure might also have 
taken place by which a similar result could be attained. For 
