514 Tans ley and Lnlham . — A Study of the 
through its connexion with the second in front of the node. It seems 
impossible to avoid the belief that the accessory cylinders are filled through 
these connexions when absorption exceeds transpiration, and drawn upon 
again when transpiration exceeds absorption. 
! The Morphological Status of 'Pith ’ / its Relation to Leaf -Gaps 
and Cortex. 
The questions relating to the morphological status of pith, the con- 
ception of ‘ground-tissue’ in morphology, and the like, which, in recent 
years, have been so much discussed by various anatomists, particularly 
by Jeffrey, Farmer, and Boodle, are brought prominently before the mind in 
considering the ontogeny and phylogeny of the vascular system of a form 
like Matoilia , where the axis of the rhizome is occupied successively by 
tracheids, sieve-tubes, endodermis, and ‘ ground-tissue pith/ in no less than 
three successive and complete cycles. Without attempting to discuss the 
whole of the questions that have been brought into the arena of this 
discussion, we cannot help expressing our belief that Jeffrey’s theory of 
the intrusion of cortex into the stele is without foundation on developmental 
and comparative grounds, and has at best a merely metaphorical value ; 
while Farmer’s view 1 , that the only valid distinction is that between vascular 
(in the wide sense) and non-vascular tissues, while perfectly good, and, so 
far as we are aware, never disputed since the time of Sachs and De Bary, as 
a fundamental physiological classification of the internal tissues of a vascular 
plant, yet radically ignores the morphological problems with which the 
anatomist is confronted when he is endeavouring to trace out the evolution 
of the tissues of vascular plants 2 . 
In our view, the pith is morphologically an entirely new tissue, formed 
in the centre of the stele, in place of vascular tissue, which preceded it in 
ontogeny and phylogeny, but is always, in the Ferns, separated from the latter 
by endodermis. The fact that its histological characters are very similar 
to, or even identical with those of the cortex, gives us no information as to 
how the pith arose in the course of descent, which is the essence of the 
morphological question ; while the general continuity of pith with cortex 
at the leaf-gaps, and the phenomena of ‘ endodermal ’ or ‘ ground-tissue 
pockets ’ considered as probable forerunners of a continuous ground-tissue 
pith, although they do furnish us with good evidence of stages in the phylo- 
genetic origin of pith, at least in certain cases, do not take us one step 
towards a demonstration of the cortical origin of pith, but merely establish 
1 Farmer and Hill (’02), p. 392. 
2 Farmer’s view that tissues cannot be treated as morphological entities in the same way 
as members of the plant body (op. cit.) is interesting, but involves questions far too wide to be 
discussed here. It appears to strike at the root of the investigation of tissue morphology on 
evolutionary lines, a subject which has been pursued with so much success in this country during the 
past decade. 
