440 SYSTEMATIC BOTANY. 



motionless ; Derbes, however, asserts that he has observed them moving 

 like undoubted spermatozoids. The antlieridia are generally found in 

 distinct plants from the spores, and the tetraspores in a third series of forms 

 of the same species. 



Where the sexuality of the Algse has been ascertained, we meet with 

 the process of fecundation under three different forms, and these forms in 

 subordinate modifications. The three forms of the process are : Conju- 

 gation, or complete union of a sperm-cell and a germ-cell, originally 

 undistinguishable from each other by visible structure, occurring in Dia- 

 tomacese and some Confervoidese ; Fecundation of naked germ-corpuscles 

 by ciliated spennatozoids, which in the Confervoideae occurs within the 

 parent cell of the spore, and in Fucaceai after both the germ-corpuscle 

 and the spermatozoids have been cast off by the parent ; and Fecundation 

 of naked germ-cells by motionless ovoid or globular spennatozoids through 

 the medium of a special tube or trichogyne, as in Rhodospermese. The 

 importance of these phenomena to the whole theory of reproduction in 

 plants renders it necessary to give a particular account of the processes as 

 occurring in certain well-ascertained cases. 



Conjugation. In Diatomaceee (including the Diatomece and Desmidiece], 

 the ordinary mode of multiplication of the plants is vegetative propaga- 

 tion, by division, resulting either in the formation of connected " families " 

 of cells (fig. 503, C) or of an increased number of separate cells, or by the 

 extrusion of zoospores,^ which are developed into new cells or cell-families 

 (fig. 503, B, a). This kind of propagation goes on actively for a time 

 under favourable circumstances ; and the mere " division," at least, may 

 be compared to the vegetative development of more complex plants. 



But at certain epochs this mode of increase is exchanged for another 

 kind, in which we have cooperation of two originally distinct cells to 

 produce the new one, indicating that it is a phenomenon of sexual repro- 

 duction, while at the same time there is no external evidence of difference 

 in the concurrent cells. The genus Closterium (fig. 503, B, c) is multi- 

 plied vegetatively by division, Qvfissiparous propagation ; at certain stages 

 of existence, however, the cells which appear as if about to divide approach 

 in pairs, and, a fracture of the external cell-membrane having taken place 

 at the usual line of division, the contents of each cell, bounded by a pri- 

 mordial utricle, escape, come into contact with each other, and become con- 

 fluent into a mass which assumes a rounded form (fig. 503, B, d). This 

 round body becomes coated by a cellulose coat, and ultimately by a second, 

 more internal. Its contents change from a green to a brown or yellowish 

 colour ; and the globular cell remains after the two empty parent cells 

 have decayed. This globular body, which passes through a stage of rest 

 before germinating, is sometimes called a sporange, not a simple spore, 

 since its contents appear to become segmented and divide into a num- 

 ber of independent germs when the structure recommences active deve- 

 lopment. 



An analogous conjugation of two cells takes place throughout the 

 Desmidiece, and it has also been observed in many Diatomece ; in all cases 

 the product is a resting sporangial cell or frustule, i. e. a cell possessing 

 more than one firm coat, which produces two or more germs when about 

 to throw off these coats to develop into a new plant of the form of the 

 parent. Conjugation exhibits many minor variations in the groups of 



