On Descriptiotis of Vascular Structures. 
1 1 i 
cells (originally xylem-parenchyma or parenchyma derived from 
potential tracheides), and there is no question of the intrusion of 
cortex, as there are no leaf-gaps. 
The necessity for the supply of a definite amount of water to 
the leaf-traces, as a function of the xylem of the protostele, appears 
in combination with other factors to have regulated the mode of 
differentiation of the latter tissue, and does not directly affect the 
cortex. Hence, as portions of xylem are easily replaced by paren¬ 
chyma, it certainly appears more probable 1 that the protostele 
itself should have become converted into vascular and parenchyma¬ 
tous portions than that an intrusion of cortex into the stele should 
have taken place. Granting this, the dictyostele together with the 
parenchyma at its centre and that forming the leaf-gaps may be 
regarded as the morphological equivalent of the protostele 2 . 
It may be urged that a probable morphology based on con¬ 
siderations of this kind is of no importance, but many things 
require investigation before we can hope for a firm foundation for 
the morphological treatment of vascular tissues, and what is at 
present important is that one should not build up a spurious 
morphology with no basis at all, or without stating its basis. 
Having discussed some points connected with the morphology 
of vascular tissues, we may now return to the subject with which 
we started, and enquire how far the two methods of description (up¬ 
ward and downward sequence) fall in with the theory stated above. 
We will assume the morphological views, just stated, as correct and 
also the theory suggested above, that many advances in complexity 
originated at the node, and spread afterwards downwards through 
the internode. On these suppositions, both in the seedling and in 
a mature stem such as that of some Gleichenias , in which the 
structure is not uniform throughout the internode, the acropetal 
method of describing the tissues gives one their morphology, tissues 
within the stele being taken as belonging to the stele; while¬ 
tracing the tissue downwards leads more to a description of the 
phylogenetic history of the tissues expressed in physiological terms, 
e.g., in the case of Gleichenia referred to above, a downward 
1 For the above reasons, and for others, which cannot be 
quoted here. 
2 Instead of excluding the parenchyma from the stele, as has 
been done by Farmer and Hill, loc. cit. The view given above 
in the text is that which has been adopted by Schoute (Die 
Stelar Theorie, 1902). 
