THE NATURE OF SEXUALITY. 
897 
It is only in a comparatively small number of cases, and in plants of very 
simple structure, like the Desmidieae, Mesocarpeae, and Pandorineae, that the two 
cells which coalesce are alike in their mode of production, size, form, and behaviour 
when coalescing; and even in these cases they probably differ internally, since it is 
difficult to explain on any other hypothesis the necessity for their union into a pro- 
duct capable of development (the Zygospore). In some other Conjugataä, as Spiro- 
gyra, this internal differentiation is exhibited at least to the extent that the contents 
of one of the conjugating cells pass over into the other, the contents of which remain 
stationary. But usually, as in most Algae ( Vaiicheria^ (Edogonium^ Coleochcefe, Fucus, 
&c.), and in all Characeae, Muscineae, and Vascular Cryptogams, a great variety 
of differences are manifested between the sexual cells as to size, form, motility, 
mode of production, and the share they take in the formation of the product of 
the union. This differentiation presents, especially in the Algae, a most complete 
series of gradations between the conjugation of similar cells and the fertilisation 
of oospheres by antherozoids, any boundary line between these two processes 
being unnatural and artificial. The difference also between the sexual cells is de- 
veloped only gradually and step by step, like the external and internal differentiation 
of plants ; and it is this that renders it probable that in the lowest forms of the 
vegetable kingdom, as in the Nostocaceae, no process at all of this kind exists, or 
that at all events there are plants of extremely simple structure in which no such 
process occurs. 
Wherever there is an evident external difference between the two sexual cells, 
one behaves actively in the union, and loses in the process its individual existence, 
the other behaves passively, absorbing into itself the substance of the active one, and 
furnishing by far the larger proportion of the first materials for the formation of the 
immediate product of the union. The former is termed the ?nak cell or anlherozoid, 
the latter the female cell or oosphere. 
These most essential features of the sexual process may also be recognised in 
the fertilisation of the Ascomycetes and Florideae, although the external appearance 
of the female organ, the carpogonium, on the one hand and of the male organ 
(in certain Ascomycetes at least) on the other hand, are strikingly different from 
those which occur in any other class of plants. 
The usual condition of the female cell during the sexual process (except in the 
Ascomycetes and Florideae) is that of a naked primordial cell (oosphere), formed 
either by simple contraction of the protoplasm of a cell previously enclosed within a 
cell-wall (as in the oogonium of Vaucheria, (Edogomum, and Coleochcete, and in the 
archegonium of Muscineae, Vascular Cryptogams, and Gymnosperms) or by the 
division of the protoplasm of a mother-cell combined with contraction and rounding 
off of the daughter-cells (as in Saprolegnia and Fucaceae), or by free cell-formation 
(as in the embryo-sac of Angiosperms). In all these cases the oosphere is spherical 
or ellipsoidal, except that in the Angiosperms it is sometimes elongated ; in general 
its form is the simplest that the vegetable cell can assume. The rounding off is not 
connected with any internal differentiation ; at least where any internal differentiation 
is exhibited (as in the formation of chlorophyll and the granular contents in (Edo- 
gonium and other Algae), the phenomenon is a secondary one in the process of 
fertilisation. The oosphere is never actively motile, even when, as in the Fucaceae, 
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