314 PROF. F. 0. BOWER ON APOSPORY AND ALLIED PHENOMENA. 
side of the midrib, and the bud arises on the side of the sorus nearest to the margin; as 
it develops, the leaves which are formed curl round the margin, and in older stages it 
appears as though the buds were seated on the upper surface of the frond; this is, 
however, not really the case (fig. 42). By means of such a case as this, we see that the 
formation of sporophoric buds from the sorus is not necessarily associated with sporal 
arrest, and we may regard this as indicating a graduation towards those common and 
well-known cases of formation of sporophoric buds at various points on the fronds of 
Ferns quite remote from the sori. 
Conclusion. 
Maintaining the use of the term “spore” in its narrower sense, as defined by Sachs, 
we may now proceed to review the facts above detailed, and to place them in relation 
with others already familiar to botanists; it may, however, be again remarked, in passing, 
that the adoption of this narrower definition excludes from discussion the large majority 
of Thallophytes, since in most cases the homologies of their asexual reproductive organs 
are not clear; it is not the object of this paper to enter upon a general discussion of the 
homologies of the asexual reproductive organs of the Thallophytes, which would only 
confuse and overshadow the points before us. Accordingly the application of the com- 
parison to the Thallophytes will be, with but one exception, completely omitted *. 
It will be well to take first into consideration the partial or complete arrest of 
development of the spores themselves, and their consequent loss of reproductive function : 
it will be remembered that to this the term “ sporal arrest" was applied, while it was 
pointed out that this may occur irrespective of the presence or absence of those substi- 
tutionary vegetative growths which so often accompany it. Іп the case of the Fern first 
described (Athyrium F.-f., var. clarissima) it has been shown that in the large majority 
of cases the development of the spores does not proceed beyond the appearance of the 
archespore, though in some instances divisions appeared in this cell in the normal 
manner; but though the formation of an annulus has been noted more than once, and 
though in one case even spore-mother-cells were seen, no mature spores have been found 
to be produced by this plant ; it is, then, an example of complete sporal arrest. Passing 
to Polystichum angulare, var. pulcherrimum, the case is here very similar, the develop- 
ment of the sporangia being arrested before the production of mature spores; thus this 
Fern is also an example of complete sporal arrest. Of the remaining three cases above 
described, not one shows a complete arrest, spores which are apparently mature having been 
found in each; still, in the subvarieties of Athyriwm at least, a large proportion of the 
sporangia are arrested in their development, and it is only the minority which appear to 
atcain maturity. Comparing these facts with others already well known, we learn that 
sporal arrest is a wide-spread, though never a very common-place phenomenon. As 
examples, the following may be cited: the case of Equisetum litorale, Kuehlewein, 
described in detail by Mildet; in this plant, which, it is suggested, may be a hybrid, on 
* Compare McNab, Scientif. Proc. Roy. Soc. Dublin, iv. (1885) pp. 451-454 and 466-469. 
T Monographia Equisetorum. Dresden, 1865. 
бы кан 
