THE NATURE OF SEXVALITT. 
«99 
The product resulting from the sexual process is usually a new individual which 
has no longer any organic connexion with the mother-plant and is not united with 
it in growth. This is the case in the Muscinese, where the sporogonium, and in 
Phanerogams, where the embryo, is nourished by the mother-plant, but there is no 
actual continuity of tissue between it and the latter. The case is quite different in 
the Ascomycetes {e. g. Lichens, Eurotium, and Erysiphe) and Florideae, in which the 
female organ itself or certain cells connected with it are stimulated by fertilisation 
to produce new shoots from which results a fructification containing spores ; and it is 
only after the completion of this complicated vegetative process brought about by the 
sexual union that the spores are set free, and produce new individuals independent 
of the mother-plant. 
The reproductive cells of the same plant do not differ merely externally ; the 
inability of either to originate by itself a new course of development, while the two 
together produce an organism capable of germinating, shows that the properties of 
the two are complementary to one another. The sexual differentiation, or difference 
between the male and female cells, which is neutralised by the act of fertilisation, 
has been preparing for a longer or shorter time ; the product which is the result of 
fertilisation owes its formation to the neutralising of the sexual difference. In the 
Conjugatse and other families where the sexual difference is extremely small or even 
imperceptible, the preceding processes of development are also alike; the mother- 
cells of the two kinds of reproductive cells even to the earliest stage of development 
do not differ externally. But where the sexual difference is greater, it is fore- 
shadowed in the preceding processes of development. Thus the mother-cell of the 
antherozoids of (Edogonium differs in form from that of the oosphere ; and this is 
especially seen in the development of the Q^dogonieae with ' dwarf males.' In 
Vaucheria the branches which subsequently become antheridia differ at an early 
stage from those which form the oogonia. The sexual differentiation of the Cha- 
raceee is inaugurated long beforehand in the great difference in the development of 
the antheridia and carpogonia, the position of the two organs on the leaf being also 
different. In the Muscineae and Vascular Cryptogams again preparation is made 
for the production of the antherozoids and oospheres in different ways by the 
formation of antheridia and archegonia. But this preparation is not confined to 
the difference between the organs which immediately produce the reproducdve 
cells ; in many classes of plants it even goes back so far that the entire plant 
developes as a male or as a female plant, producing only male or only female 
reproductive organs. This occurs in some Algae, Characeae, Muscineae, and in 
the prothallia of some Vascular Cryptogams. 
The fact is very remarkable that this preparation may be carried back in the 
development of the individual even beyond the limit marked by the alternation of 
generations. In the Algae, Characeae, Muscineae, Ferns, and Equisetaceae, the nature 
of the alternation of generations is such that the sexual differentiation is developed 
in one of the generations, while it is neutralised in the succeeding generation. In 
these cases therefore we have a sexual and an asexual generation in the course of 
the development of the same individual ; the asexual generation is the product 
of the neutralising of the sexual differentiation of the sexual generation. The two 
generations, especially in Muscineae and Vascular Cryptogams, differ essentially from 
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