24 
Proceedings of the Royal Society of Edinburgh. [Sess. 
solenostelic : transverse sections of them drawn to the same scale are 
shown in fig. 9 : 2, 3. Dipteris conjugata is the larger, and shows only 
a simple solenostele : Matonia pectinata is the smaller, and shows three 
concentric rings. A further example is seen in fig. 13, A, B, where in stems 
of equal size that of Cibotium is a simple solenostele, that of Hemitelia is 
a dictyostele with medullary strands. A comparison of drawings illus- 
trating the origin of the solenostele in individual plants of Gleichenia 
pectinata , Loxsoma, and Histiopteris incisa, shows that no absolute size- 
limit rules for them, though in each individual a great increase of size 
accompanies the structural change. There need be no surprise that this 
difficulty should arise, for it is common knowledge that the character- 
istics of plants, and ultimately of their protoplasts, differ. One is more 
resistant to temperature, or drought, or insolation than another. The 
incidence of the “limiting factor” will depend upon the specific perme- 
ability of the endodermis, and particularly of its protoplasts. It seems 
probable that this may vary from plant to plant without any visible 
difference of structure, just as much as other characteristics of the pro- 
toplasts do. It is only when the specific permeability of the endodermis 
has been measured for any plant that we can reasonably expect to be 
able to state in terms of exact measurement where for it the incidence 
of the limiting factor may come. With our present knowledge it is 
only a rough suggestion of the existence of a critical point that can 
be indicated. But this gives a sufficient ground for recognising the 
underlying principle of similar structures as applicable to vascular tissues. 
It appears to have determined certain of their peculiarities, which have 
hitherto appeared as strange and unexplained phenomena. It is believed 
that by directing attention to the sheaths surrounding the vascular 
tracts, their presence or absence, their structure and permeability, and their 
relations however roughly to absolute size, a better understanding of the 
vascular systems of plants, and of the ferns in particular, will be obtained 
than by the most carefully drawn comparisons of mere formal anatomy. 
Size must then be considered not only in its bearing on external form and 
the strength of materials, but also as it may tend to modify, or even in 
some cases to rule decisively the structure and disposition of internal 
tissues. 
As far as I am aware only one definite attempt has hitherto been made 
to correlate size with internal structure in the massive plant-body. 
Professor Compton, in a very remarkable comparative study of the 
seedling structure in the Leguminosse {Linn. Journ., vol. xli, p. i, 1912), has 
shown that in determining the level of transition from root-structure to 
