23 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
increase within has already so stretched and disorganised the endodermis 
that it can no longer act as a physiological barrier. It would be worth 
while to examine it carefully as to the manner of its functional change, 
from the point of view here suggested, and to trace in detail the disruption 
or disorganisation of the sheath. But such observations would not be easy. 
It is only in cases where the endodermis is marked by special structure 
that this can be readily seen. An example has been worked out by Miss 
Mann in the large roots of Dracaena, which show secondary thickening. 
The effect of internal expansion of tissues is shown in bursting asunder the 
thickened cells, and communication is thus established between the cortical 
and stelar systems by tracts of ventilated parenchyma. Once the 
endodermal barrier is interrupted, communication is continued radially 
inwards along the medullary rays of Dicotyledons and Gymnosperms, as 
Strasburger showed long ago. It is in some such way as this that the 
problem is solved for trunks and roots of the largest size The same holds 
also for the dendroid stems of Monocotyledons, such as Dracaena and 
Cordyline. In them tangential sections show that intercellular spaces run 
radially inwards in the hardened tissues that lie between the secondary 
strands. 
The success with which the flowering plants have thus met the 
requirements of increasing size stand in strong antithesis to the difficulties 
with which the ferns have had to contend. Doubtless the strictly enclosed 
conducting system of ferns is a more effective means of transport for 
plants so dependent as they are upon a constant water-supply. But the 
problems which increasing size has raised in them could only be solved 
by extraordinary modifications of structure : and that seems to be the 
physiological explanation of those remarkable vascular systems which 
they show, often in clearly marked succession in their individual develop- 
ment. The ontogenetic evidence is in fact more weighty than that from 
comparison. 
Those who pursue sciences of exact measurement may expect me to 
give precise statements as to the actual size of stele which will be possible 
with a certain structure, and of the exact dimensions at which the 
“ limiting factor ” will become operative : that at which either a new form 
of stele must be adopted, or a change be made in the visible structure of 
the endodermis, so as to alter its permeability. At present it is not 
possible to put forward measurements of that nature. The difficulty is 
illustrated by the drawings, all to the same scale, in figs. 9 and 17 ; and 
especially by the cases of Dipteris and Matonia, two genera of allied 
ferns, with many analogies of structure. They are both rhizomatous and 
