102 Davie . — The Development of the Sorus and Sporangium 
still is present as the sorus lengthens (Fig. 5). When the receptacle grows 
out further as the narrow stalk of the sorus, the curving over of the indusial 
flap is very marked. It is then curved back on one side, until the portion 
by which it is attached to the 
stalk is almost parallel to 
the surface of the leaf (Text- 
fig. 1). The earliest indica- 
tions of this recurving at the 
side are seen in Fig. 4. 
Of the second flap there 
remains in the older sorus 
only a mere knob, seen on 
the left side of Text-fig. 1, 
at the top of the stalk. The 
extreme tip of the indusium 
is bent in (Text-fig. 1), and 
between the recurved tip 
and the stalk is a narrow slit, 
which appears on the surface 
of the sorus as an elongated 
pore. This slit marks the 
edge of the ‘ cup ’ forming 
the indusium. In a trans- 
verse or oblique section of 
the mature sorus the two 
edges of the ‘ cup ’ may be 
seen (cf. PI. Ill, Fig. 6, and 
Ann. of Bot., xxvi, PI. XXIX, 
Fi g- ! 5 )- 
These sections through 
the sorus show that the in- 
dusium arises as a cup round 
the receptacle, that one side 
of the cup is strongly de- 
veloped and curved over the 
top of the receptacle, while 
a small portion on the other 
side is suppressed in de- 
velopment. On this side at 
maturity the edges of the 
remainder of the indusium 
curve in to form a pore. 
The receptacle, at first 
only slightly raised above the 
surface of the leaf (Fig. i), 
Text-fig. i. Vertical section through a semi-mature 
sorus, showing the 1 stalk ’ fully developed. The main 
indusial flap now completely covers the wide receptacle and 
is incurved at its tip. The small knob opposite to this 
incurved tip represents the remains of the laggard portion of 
the indusium. The ‘kink’ shown in PL III, Fig. 4 has 
developed as the series of cells on the right of the sorus 
and now extends almost at right angles to the ‘ stalk ’. The 
youngest sporangia occur among the stalks of the oldest at 
the summit of the receptacle ; quite young sporangia appear 
at its margins. The sequence of sporangia is thus at first 
basipetal, and later mixed, x 65. 
