ZYGOSPORES. 251 



reproduction takes place by means of conjugation, the essential characteristics 

 of this process being that the two cells which take part in it are alike, and 

 produce, by the coalescence of their protoplasmic contents, a cell of peculiar 

 form, the Zygospore, which usually remains for a time dormant, and is then termed 

 a resting-spore. Conjugation is the simplest form of sexual reproduction, and 

 the morphological characters of the plants belonging to this class are, as might 

 be expected, much simpler than of those which constitute the succeeding classes. 

 The mere fact of sexuality does nevertheless show an advance on the mode 

 of reproduction of the Protophyta, and the Zygosporeae manifest in consequence a 

 higher degree of organisation, and present the transition from the non-sexually 

 propagated Protophytes to those forms of Thallophytes in which reproduction is 

 sexual in the strict sense of the term. 



Like that of the vegetative organs, the form of the organs of conjugation 

 varies greatly in the different sections of this class; the zygospore is sometimes 

 produced by the conjugation of naked zoogonidia, sometimes of highly developed 

 cells belonging to the thallus, sometimes of special branches which do not occur 

 elsewhere in the thallus. One of the most remarkable phenomena connected with 

 sexuality is the formation of Auxospores in the Diatomaceae, which, as far as our 

 present knowledge goes, takes place in some cases by actual conjugation, but in 

 others, as Schmitz has shown, by the simple approximation of two cells without 

 any coalescence or actual contact, an interchange of substance taking place pro- 

 bably by diffusion. 



The plants comprised in this class differ greatly in the structure of their 

 vegetative body; and we are at present acquainted with but few intermediate 

 transitional forms connecting the various sections belonging to it. This evi- 

 dently arises from the fact that it is only of late years that the process Of 

 conjugation, previously known only in the Conjugatse, has been studied in the 

 Pandorineae, Zygomycetes, and other families. It may be expected that the further 

 investigation of the zoogonidia of a large number of Algse will show them to be 

 conjugating sexual organs; and it is even possible that among plants which are 

 very nearly allied some produce zoogonidia which actually conjugate, while in others 

 the corresponding cells do not usually conjugate, but proceed to a further develop- 

 ment, or, in other words, are propagated parthenogenetically. Some such phe- 

 nomenon is indicated in the formation of the auxospores of the Diatomaceae already 

 mentioned; and, on the other hand, we find forms, like the Hydrodictyese^ in which 

 no conjugation of the zoogonidia has hitherto been observed, although they are 

 nearly connected, in their morphological characters, with the Pandorineae in, which 

 this mode of reproduction does occur. I do not therefore hesitate in assigning the 

 Hydrodictyeae a place among the Zygosporeae. The question is more difficult 

 whether, in addition to the Zygomycetes, the Chytridineae and Myxomycetes should 

 also be included in this class. In the Chytridineae it is probable that some of 

 the zoogonidia conjugate, although this has not hitherto been observed^. In the 



' [The conjugation of microzoogonidia was observed by Suppanetz in 1873, Rostafinski, Mem. 

 Sec. Sc. Nat. de Cherbourg, 1875, vol. XIX, p. 152.] 



^ [Novakowski has discovered sexual reproduction in Polyphagus EnglencB. Cohn, Bcitr. 3. Biol. 

 1876, Bd. II. See also infra, p. 264.] 



