Bower.—Studies in the Phytogeny of the Fi lie ales. 513 
at that lower level in the mesophyll of the leaf, it extends past the limiting 
vein of its own areola, thus crossing into the next. This is what has been 
seen to happen occasionally in Cheiropleurict ; but it is a much more pro¬ 
nounced feature here in the partially fertile leaf of Platycerium. Still more 
is it so in the normal fertile area, as is shown in Fig. 14. Here the recepta¬ 
cles are much more elongated, and occasionally branched, and follow a 
distinctly parallel course. Their connexion with the main venation is as 
Text-fig. 13. Part of a lamina of Platy- 
cerium aethiopicum , which is only half-fertile, 
i. e. with the sori isolated and small: for com¬ 
parison with Fig. 12. x 3. 
Text-fig. 14. A similar drawing from a 
fertile leaf of Platycerium angolense. The position 
of the sori is upon the distal ends of blind veins ; 
these elongate and enlarge as in Cheiropleurict) 
and pass to a lower level in the mesophyll: they 
extend frequently across the limits of the areola 
of the main venation which lies in a higher 
plane, x 3. 
before; but once they have passed into the lower plane in the thick and 
fleshy leaf, they may extend to great length, and cross not one only, but 
several areolae, and branch in their course. And thus there is constituted 
that second vascular system of the fertile area recognized by Mettenius. 
The appearance in transverse section is shown in PI. XXV, Fig. 16, in 
P. Willinkii, from which it is seen that the two vascular systems extend in 
quite different planes, and that the receptacular strands are in close relation 
to the insertion of the sporangia. 
While we may thus feel a special interest in this elaborate extension 
of the receptacle, and in the prolongation of its tracheidal system to form 
a sort of vascular system of its own, it may be recalled that such a develop¬ 
ment does not stand alone. In various types of Ferns the vascular system 
of the receptacle is liable to spread beyond the restricted area of the primi¬ 
tive sorus. It has been seen how in Saccolomct the marginal sori are isolated; 
