11 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
barrier and the completion of a common ventilating system for cortex and 
pith. The “ limiting factor ” was met by interruption. This is in point of 
fact a more effective device than that seen in the enlarged protosteles, 
which maintained their endodermis but enlarged its area. 
Lang has shown in the living Ophioglossacese a method of resolving the 
difficulty similar in effect to that of the Osmundacem. In Botrychium 
and Helminthostachys the young plant has a complete endodermal barrier, 
as in other ferns, shutting off the vascular system from the surrounding 
cortex. But as the plant advances, the conical stele enlarges, and a pith is 
Fig. 8. —Osmundites Carnieri , Schuster. Arrangement of meristeles. 
The endodermis is shown by dotted lines. After Kidston and 
Gwynne- Vaughan. 
formed which serves for storage, and contains starch. Intercellular spaces 
appear in it, but the internal ventilation-system is at first wholly shut off 
from the cortical, and it remains so till the plant is well advanced. 
As the stem enlarges free communication is established by foliar gaps, 
which naturally open outwards to the cortex, but here they are always 
open inwards also to the pith. This has the disadvantage of laying open 
the conducting tract, and destroying the completeness of endodermal con- 
trol ; but it resolves the difficulty of communication between the outer and 
the inner tissues, which becomes more acute as the stem enlarges. The 
advantage gained is probably greater than the disadvantage that follows. 
What is thus seen in less complete form in Botrychium and Helmintho- 
stachys is carried much further by the genus Ophioglossum. Here the 
endodermis is discarded early. The stele dilates with a distended pith, 
