802 PHENOMENA OF SEXUAL REPRODUCTION. 



coalescing^; and even in these cases they probably differ internally, since it is diffi- 

 cult to explain on any other hypothesis the necessity for their union into a product 

 capable of development (the Zygospore). In some other Conjugatse, as Spirogyra, 

 this internal differentiation is exhibited at least to the extent that the contents of 

 one of the conjugating cells pass over into the other which remains stationary. 

 But usually, even in many Algae (as Vaucheria, CEdogonium, Coleochaete, Fucus, &c.) 

 and Fungi (Saprolegnia), and in all Characeae, Muscineae, Vascular Cryptogams, and 

 Phanerogams, 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 form- 

 ation of the product of the union. This differentiation presents, especially in the 

 Algae and Fungi, a most complete series of gradations between the conjugation of 

 similar cells and the fertilisation of the oosphere by antherozoids, any boundary line 

 between these two processes being unnatural and artificial. The difference also 

 between the sexual cells is developed only gradually and step by step, like the ex- 

 ternal 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 7Nak or sperm-cell, the 

 latter the/emale or germ-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 sexual organs, the ascogonium and trichophore on the one hand and the 

 antheridia 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 (the oogonium of Vaucheria, CEdogonium, and Coleochaete, the central cell 

 of the archegonium of Muscineae and Vascular Cryptogams) 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 

 corpusculum of Coniferae .? and the embryo-sac of Angiosperms). In all these cases 

 the germ-cell is spherical or ellipsoidal, except that in the Angiosperms it is some- 

 times 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 



^ See De Bary, Die Familie der Conjugaten, Leipzig 1858, p. 57; Pringsheim, Monatsber. der 

 Berlin. Akad. Oct. 1869, Paarung der Schwarmsporen [Ann. des sci. nat. 5th series, 1869, vol. XII, 

 pp. 191 and 211 ; De Bary, ibid. p. 208] ; Pfitzer, in Hanstein's Botanische Abhandlungen, 1871, 

 Heft II, p. 70 et seq. 



' De Bar)', Beitrlige zur jNIorphologie und Physiologic der Pilze, Frankfort, Heft III, at the end. 



I 



