PROF. F. O. BOWER ON APOSPORY AND ALLIED PHENOMENA, 309 
in material taken in August), the extreme apex consists of cells with relatively thin walls 
and more abundant protoplasm, and they show signs of repeated division. It was not 
possible, however, to refer the development in its origin to any one individual cell, nor in 
early stages of development was any single initial cell to be noted; the growth appears 
rather to be a general one, distributed over a considerable and not definitely limited area 
of the apex of the pinnule. The whole process is carried on in a region above the extreme 
ending of the vascular bundle, which accordingly takes no part in the development of 
the type (A) (Plate LVIII. figs. 18-20). Until it attains a considerable size, the prothalloid 
outgrowth thus formed remains as a flattened expansion, only one layer of cells in thick- 
ness; but, subsequently, a ** cushion," or more massive region, is formed as in a normal 
prothallus, and on it archegonia are produced. Antheridia are also present in some cases, 
though apparently not in all (fig. 21). During this development the tips of the pinnules, 
as well as the prothalloid outgrowths themselves, assume the most complicated curva- 
tures, so that it is often difficult to observe the details under the microscope without more 
or lessinjury to the object. It will be recognized that this type (A) of development of the 
prothallus from the apex of the pinnule, or of one of its segments, was the only one 
hitherto described by myself and others as occurring in Polystichum angulare, var. pul- 
cherrimum ; also that it more nearly follows the course of normal development of а pro- 
thallus from the spore than the type (B) now to be described, since a flat expansion is 
formed first in type (A), the more bulky “ eushion " only appearing subsequently. 
As above stated, the type (B) differs from (A) in its mode of origin, the prothalloid 
growths being derived from the surface of the pinnule, not from its apex. Moreover, 
from the first, these prothalloid growths are of a massive character, and often assume very 
peculiar forms. In the material taken in August cases were to be found similar to that 
represented in fig. 22, where a slight hemispherical swelling was to be seen on the surface 
of the pinnule, immediately above the ending of one of the nerves (vd.). Оп cutting 
sections through such specimens, it is clear that the swelling is due to outgrowth of the 
superficial tissues at a point removed from the apex of the pinnule, that the outgrowth is 
at first of a purely parenchymatous nature, and that, at first at all events, the vascular 
bundle takes little or no part in the formation (fig. 23). 16 appears that the outgrowths 
may arise either on the upper or the lower surface of the pinnule. : As they grow older 
they assume an elongated cylindrical or obconical form, maintaining throughout their 
massive character. An investigation of their structure by means of sections shows that, 
in some cases at least, the vascular bundle is continued with a reduced structure for a 
short distance into the prothalloid outgrowth (Plate LIX. fig. 24), though it soon dies out. 
A transverse section at a short distance above the point of insertion on the pinnule accor- 
dingly shows a simple undifferentiated character of the tissues (fig. 25). A poqulineity of 
internal structure is to be noted in sections, either transverse or longitudinal, at points 
near the apex; these show the presence of heterocysts (fig. 26), usually surrounded by a 
comparatively small-celled epithelium. Having assumed a massive, elongated obconical 
form, and often very contorted (figs. 27-29), these growths of the type (B) put out nume- 
rous rhizoids, similar to those of a normal prothallus, while, occasionally, irregular mas- 
sive outgrowths appear laterally as in fig. 29, Тһе extreme apex is always PORK and, 
SECOND SERIES.—BOTANY, VOL. 11, ЗА 
