308 PROF. F. О. BOWER ON APOSPORY AND ALLIED PHENOMENA. 
has already been the subject of previous notices, by Mr. Wollaston *, who has described 
and figured it, by Mr. Druery +, and by myself $. 
Over and above the peculiarity of the formation of prothalloid outgrowths from the frond, 
Р. angulare, var. pulcherrimum differs from the typical P. angulare in two points— 
first, in the contour of the pinnule, the segments on the upper or acroscopic side of the mid- 
rib being normal, while those on the lower side are flabellulate, elongated and narrow, and 
curved upwards (compare Mr. Wollaston's figures); secondly, on the specimens which 
have come under observation I have never seen a mature sporangium with spores; on 
fronds which produce sori in the normal position, with a normal indusium, an examina- 
tion of the sporangia discloses the fact that their development has been arrested ( Plate 
LIX.figs. 32-36). The arrest is, in some cases, at a period when the sporangium consisted 
of but a few cells ; in other, but comparatively rare, examples it is subsequent to the for- 
mation of the annulus; but in no case have they been seen to contain spores even 
approaching to the mature condition. These being the results of examination of a very 
considerable number of sporangia, it may with safety be concluded that at least the speci- 
mens observed were examples of complete sporal arrest; and, further, that the pro- 
thalloid growths to be described below do not owe their origin to spores. This conclusion 
is amply justified by anatomical investigations, which point distinctly to their origin by 
direct vegetative out-growth from the sporophore. 
It has been found, however, that though the prothalloid developments thus originate 
by direct vegetative outgrowth from the sporophore, there is in this plant considerable 
variety in the details of the process. According to the mode of their origin, at least four 
types of prothalloid development may be recognized and described. These may be ranged 
under two main heads, according as they arise :—(I.) entirely separate from the sori, or 
(11.) in connection with a sorus. We may consider first those which fall under the 
former head, viz. those in which the prothalloid outgrowths arise at points quite apart 
from the sori. Such developments may even be found on fronds which bear no sori at all, 
and this was conspicuously the case in specimens supplied by Mr. Druery from a plant 
from Chard. This formation of prothalli from the frond, and apart from the sorus, 
appears at or near the extreme apex of the pinnule, or one of the smaller segments of it, 
and there may accordingly be distinguished two subtypes, which may be named (A) and 
(B). As (A) may be recognized that in which the development is from the extreme apex 
of a pinnule or of one of its segments, and the result is in the first instance a thin flattened 
expansion (figs. 19, 21); type (B) originates from the surface of the pinnule at a point 
opposite the ending of one of the nerves; the result is in this case a massive cylindrical 
structure, easily distinguishable from that of type (A) (figs. 27-29). 
Taking first type (A), in which the prothallus originates directly from the apex of the 
pinnule, or a segment of it, it is found that the normal structure of the apex is as repre- 
sented in fig. 17, the cell-walls being relatively thick and often sinuous in outline. In 
those cases where a prothalloid development is beginning to form (such as may be found 
+ Gard. Chron. 1885, p. 780. See plate, р. 781. 
T Proc. Brit. Nat. Hist. Soc. vol. iv. p. 3 (1884-85). 
+ Journ. Linn. Soc., Bot. vol. xxi. p. 360, plates xi. & xii. 
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