3 
Size, a Neglected Factor in Stelar Morphology. 
1920-21.] 
pass from the juvenile to the adult 
state. For the moment only the 
primary increase is meant : all second- 
ary or cambial increase may be 
ruled out of this discussion, however 
interesting its problems may be. 
Here the intention is to concentrate 
upon those problems which any land- 
living organism having no cambial 
increase must face as it passes from 
the juvenile to the adult state. It is 
the facts of ontogenetic development 
in plants without secondary growth 
which provide the most cogent evi- 
dence of the effect of increase in size 
upon internal structure. Good illus- 
trations are provided by the Filicales. 
Here the first leaves are small : the 
later leaves are successively larger. 
The stem which bears them is rela- 
tively small at its base, but in propor- 
tion as larger leaves are formed the 
supporting stem becomes progres- 
sively larger, till the adult size is 
reached. The same is the case for 
the stele that lies within (fig. 1). It is 
small at the base, and approximately 
cylindrical; but passing upwards its 
transverse section gradually increases, 
till finally in most ferns it takes one 
of those complicated forms that are 
so characteristic of the class (fig. 2). 
The form of the stem at its base, and 
of the stele within, is then not a 
cylinder but a gradually enlarging 
cone. Consequently problems depend- 
ing on the proportion of surface to 
bulk, whether of the stem as a whole 
or of the stele which it contains, will 
be progressively changing in each 
Fig. 1.— Plan of stelar construction of a juvenile plant 
of Gleichenia pectinata, after Dr J. M. Thompson, 
showing in median section the way in which the 
stele enlarges conically upwards, and widens into 
a solenostele, with leaf-gaps. a-L = the insertions 
of the successive leaves. J£« = endodermis. 
