ZYGOSPORES. 
reproduction takes place by means of conjugation, the essentia.! 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 Conjugate, has been studied in* the 
Pandorineae, Zygomycetes, and other famihes. 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. 
Soc. Sc. Nat, de Cherbourg, 1875, vol. XIX. p. 152.] 
^ [Novakowski has discovered sexual reproduction in Polyphagu^ Euglencc. Cohn, Bcitr. 3. Biol. 
1876, Bd. II, See also infra, p. 264.] 
