THE NATURE OF SEX UA LI TV, 
new plants without fertilisation and without any difference in the mode of their germination 
from that of true oospores, with this single exception, that the quiescent period of the 
parthenogenetic cells is shorter. 
Pringsheim's account of the development of the parthenogenetic forms of these 
plants which is appended here is of great interest : — ' The successive generations both 
of Saprolegnta ferax and of Achlya polyandra produced by cultivation become smaller, 
and the number of male filaments diminishes in each succeeding generation until they 
finally cease to be formed, and thus the monoecious forms become replaced by purely 
female ones.' These observations show that as the result of continued cultivation combined 
with the action of certain unfavourable conditions which accompany every attempt at 
cultivation, the formation of male sexual organs at length ceases. Possibly it is in 
consequence of the action of similar adverse conditions upon its internal constitution 
that Chara crinita ceases to form antheridia after having grown for a considerable time 
in certain waters. Possibly also these internal disturbances may affect the nature of the 
female cells, though they are developed in the usual external form, so that they are not 
sexually differentiated or only imperfectly so. In this case the effect would be one of 
the obliteration of the existing sexual differentiation, or in other words, a case of 
retrograde metamorphosis, and this is quite as conceivable as the first origin and the subse- 
quent development of sexuality. In future investigations of the subject attention ought 
to be paid to the question whether the oospheres developed upon plants of Saprolegnia 
ferax and of Achlya polyandra which bear antheridia also are capable of parthenogenetic 
development, or whether this property belongs only to oospheres developed by plants 
destitute of antheridia ^. However difficult it may be to answer this question by experi- 
ment, it must be done before it can be possible to decide whether or not the development 
of the male organs deprives the oospheres of their power of independent development, so 
that in proportion as the development of the male element diminishes the partheno- 
genetic property increases. Since we may assume that the essential object of fertilisation 
is to give to the oosphefe something which it lacks but which is necessary for its further 
development, a parthenogenetic oosphere must possess, independently of fertilisation, that 
which it requires for its further development, that is, it is not sexually differentiated, and 
this probably because the differentiation of the male element has been suppressed. 
(c) The Effects of Sexual Coalescence. Since nearly all plänts, and more especially 
the majority of Thallophytes, are capable of reproducing themselves asexually, and since 
this is the usual mode of reproduction in many species^ it may well be asked what the 
significance of sekual reproduction really is. If sexuality is merely concerned in the 
development of new individuals, it is difficult to understand why the asexual reproduction 
should not suffice. This question is of especial interest with reference to those lower 
forms of Thallophytes which reproduce themselves through many generations by asexual 
cells which may be either motile or non-motile : with reference to many Phanerogams, 
for instance, the Conifers^ it seems as if without sexuality, which induces the formation 
of the seedj no reproduction would be possible. 
The significance of sexuality is seen in quite another light when those plants are 
considered which exhibit a distinct alternation of generations, such as the Ferns, 
Equisetaceae, the Mosses, and others. In Sect. 29 of Book I, I endeavoured to show 
that the alternation of generations, wherever it occurs in the Vegetable Kingdom, 
is produced by sexuality, and that without it no such alternation is possible. In all 
cases of well-marked alternation of generations an organism which finally bears sexual 
organs is developed from a spore which has been produced asexually; fertilisation 
initiates a new process of development which closes with the development of the spore. 
Before fertilisation, there is merely the organism developed from_ the spore, the first 
or sexual generation : after fertilisation the second or asexual generation is developed 
^ [According to de Bary's recent investigations, the former of these two answers to the question 
is the correct one.] 
