7 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
staining reactions. Such evidence points to the endodermis as a selective 
screen, or even an effective barrier to physiological transit between the 
outer tissues and the conducting system. Hence the constant diminution 
of the proportion of surface to bulk as the stele increases in size becomes 
a matter of the utmost importance. A conical increase in size of the stele 
is illustrated in all ferns, as well as in other plants. It starts from the 
minute stele of the sporeling, and expands as a support for the successively 
larger leaves of the established plant (figs. 1, 2). Often the increase is rapid, 
especially in ferns with short internodes. For each plant which thus 
enlarges its stele in conical form, a limit must ultimately he reached where 
the facility for interchange through the endodermis will not suffice for the 
needs of the tissues within. This facility for interchange will then become 
a “ limiting factor.” Either some means of increasing the surface area of 
the stele, and so of increasing the means of transit, must be supplied, or 
the conical enlargement of the stele must be checked, and the later regions 
of the stele will be cylindrical. The increase cannot be continued 
indefinitely in the form of a cone. But on the other hand, any deviation 
from the simple conical form, by involution of surface or by excrescence, 
will give an increase of the proportion of surface to bulk, and thus tend to 
overcome the difficulty. We may now proceed to see how these demands 
following on increase in size have been met in the stems of ferns. 
It is generally admitted that the protostele is the most primitive stelar 
type. It is present in the juvenile stage of all ferns, and it is permanently 
retained in the adult stems of some of them. It consists of a central core 
of xylem often composed only of tracheides, as in Botryopteris cylindrica 
(fig. 5). This is surrounded by a band of phloem, followed by the 
pericycle, and finally the stele is delimited externally by the continuous 
sheath of the endodermis. No intercellular spaces have been found in the 
protostele, and the endodermis serves as a complete gas-barrier limiting the 
ventilating system of the cortex internally. Thus constructed, the stele 
receives the trace of each successive leaf, and it is important to note that 
its entry is effected without any break of continuity of the endodermal 
envelope, which thus forms a gas-tight barrier surrounding the whole 
vascular system. The protostelic structure is retained in the adult stems 
of Botryopteris, Gleichenia, Lygodium, and Gheiropleuria. It is also 
characteristic of the stems of the Hymenophyllacese, which with others are 
relatively primitive types, having stems of moderate dimensions. 
In the ferns named the stele is often minute, and never actually large. 
In Botryopteris cylindrica it is about • 5 mm. ; in Lygodium 1 mm. to 2 mm. ; 
in Gheiropleuria about 1 mm.; .in Trichomanes scandens, one of the larger 
