184 
Ethel N. Thomas. 
The whole matter is somewhat obscured by the over-lapping of 
the above stages, which nevertheless are fairly well defined. The 
trace loses its connection with the stem-stele during stage (iii.) 
Accessory Steles. 
At each node one of the internal accessory steles mentioned 
above, approaches the gap formed by the out-going leaf-trace, and 
later closes this gap by fusion with the free ends of the solenostele. 
In longitudinal section they appear as short irregular strands which 
branch and fuse, but always end blindly (Text-fig. 38). 
Fig. 38. Longitudinal section of stem. It shows the irregular form of the 
short blind internal accessory steles. 
Leaf Blade. 
The lamina of the leaf is more highly differentiated than is 
usual among Ferns, and shows many features obviously correlated 
with the exceptional habitat of this species (PI. IV., Fig. 9). 
It is markedly bifacial, the upper half being characterized by a 
large-celled hypoderm, and beneath this two layers of elongated 
closely-set palisade cells. The lower half, which occupies about 
two-thirds of the total thickness, consists chiefly of loosely-arranged 
spongy mesophyll cells with plentiful lacunae. The hypoderm on 
this side is frequently interrupted by the stomatal air chambers. 
The stomata themselves are flush with the surface, and not raised 
from it, as in some of the other species of Acrostichum examined. 
