5 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
This example of a readily intelligible case, involving elaboration of 
external form and increase of the surface thereby, is held as an important 
parallel to certain cases of surface-adjustment of internal tissues which 
are to be described later. 
In the young stems of Vascular Plants generally, and in those of the 
Ferns in particular, the conducting tracts are strictly delimited from the 
surrounding tissues by endodermis. The same is the case also for roots. 
Fig. 3. — Helminthostachys zeylanica : part of transverse section of root (Gwynne- Vaughan collection, 
slide 589 ; x 66). The endodermis, recognised by the characteristic structure of its radial walls, 
marks a boundary between the outside cortex, with large starch-grains (here above), and the 
inner conjunctive parenchyma (here below), with small grains. Drawn by Dr J. M. Thompson. 
This endodermis forms not only a morphological, but also a physiological 
boundary that is without any gap or imperfection. Its physiological 
importance consists in the fact that the structure of the endodermis places 
the contents of the conducting tract under strictly protoplasmic control. 
All the lateral walls of its cells are so specialised in substance that, instead 
of being permeable like ordinary cellulose walls, they are impervious to 
fluids. Thus all possible leakage is stopped, and the only channel of transit 
for substances into or out of the stele is under the control of the living 
protoplasts of the endodermal cells. This control applies not only to salts, 
sugars, and other similar soluble substances, but also to gases. Since in 
