THE NATURE OF SEX UAL I TV. 897 



It is only in a comparatively small number of cases, and in plants of very 

 simple structure, like the Desmidiese, Mesocarpeae, and Pandorinese, 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 Conjugatse, 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 ( Vaucherta, CEdogonium, ColeochcBte, Fucus^ 

 &c.), and in all Characese, Muscinese, 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 Algge, 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 tha 

 immediate product of the union. The former is termed the male cell or antherozoidi 

 the latter ihQ/emale 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 ColeochcBle, 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-formatiori 

 (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 CEdo- 

 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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