46 Bower.—Studies in the Phytogeny of the Filicales. VII. 
flap. When mature the sorus appears in section as a considerable area of 
leaf-surface curved downwards, and with the margin running out into the 
not highly specialized indusial flap. 
A rather more advanced condition is seen in the related species, 
P. cretica, L. (Fig. 32). Here the general relations are the same. The 
receptacle, which is broad as in P. serrulata , is only slightly convex. 
Below it lies a widely expanded commissure of tracheides suited to supply 
the large area of the receptacle, upon which the numerous sporangia of the 
mixed sorus are borne. It will be noted that the oldest sporangium 
(represented by its stalk 
only) occupies the middle 
point — as in Fig. 31, 
a , of P. serrulata . But 
such indications of the 
original gradate condi¬ 
tion are evanescent and 
inconstant, and the sorus 
soon becomes of a ‘ mixed ’ 
character, with occasional 
hairs irregularly inter¬ 
spersed. The whole is 
covered in by the upper 
indusium, which is here 
again of marginal origin. 
It shows in its basal region 
large swollen cells, which 
have the effect at maturity 
of everting it, so as to 
expose the sporangia. 
Such a sorus shows an 
advance in specialization upon those of Paesia , Pteridium , and Lonchitis . 
The specialization is more clearly shown in P. biaurita , L., where again 
the indusial flap is directly marginal (Fig. 33, a> b). It is to be noted 
that the origin of the first sporangium is delayed until the foundation of 
the massive sorus is fully laid (c). But this point is still more strikingly 
illustrated in P.podophylla, Sw., material of which was received from Jamaica 
through the kindness of Mr. Harris. The earliest stages were not ob¬ 
served, but the mature sorus shows a very wide, almost flat receptacle, 
with extended vascular commissure (Fig. 34). The sporangia are very 
numerous, with various ages intermixed., and numerous hairs distributed 
throughout. 
Fig. 34. Vertical section of the mature sorus of Pteris 
podophylla . (X85.) 
