90a PHENOMENA OF SEXUAL REPRODUCTION. 



different groups of Thallophytes. The variety of the modes in which the auxospores are 

 formed among the Diatomeae indicates that conjugation has become developed here from 

 the first quite independently of the connection of this group with the Conjugatae; and 

 in the latter group so many modes of the formation of zygospores occur that possibly 

 the development of sexuality began in several different species included within it. The 

 mode in which conjugation takes place among the Zygomycetes seems to indicate that 

 the sexual act originated independently, and it is still difficult to trace a historical 

 connection between the fertilisation of the Ascomycetes and of the Florideae and that 

 of any other group of Thallophytes. It is more feasible, as Pringsheim has already 

 suggested, to regard the fertilisation of the CEdogonieae and of the Vaucheriaceae as 

 being a further development of the conjugation of two motile cells. If, as appears 

 to be the case, a sexual act originated at different times in different Thallophytes, a 

 series of further developments should be found to correspond to each distinct origin, 

 but at present it is impossible to say from which of these origins the formation of the 

 archegonia of Mosses and of Vascular Cryptogams and that of the reproductive organs of 

 Phanerogams has been evolved. 



The question arises with reference to the simplest phases of sexuality, as to whether 

 conjugation was effected primarily in consequence of a sexual differentiation of the cells, 

 or whether conjugation preceded any sexual differentiation and that this only made its 

 appearance as a secondary phenomenon when plant-forms had become more highly deve- 

 loped ^ The former of these two alternatives is supported by those phenomena which 

 indicate a mutual action at a distance of the conjugating cells and which have been fre- 

 quently mentioned by observers as a sort of mutual search. The latter is borne out by the 

 consideration that the conjugating processes are developed from those points only of the 

 mycelium, in Zygomycetes and in many Ascomycetes, at which they are in contact. It 

 is known that similar, but quite infertile, connections occur between mycelial filaments, 

 and there are good grounds for believing (Sect. i6) that these phenomena of growth are 

 induced by pressure ; and therefore it may be reasonably inferred that the growth of the 

 conjugating filaments and the formation of zygospores is a further development of the 

 sterile coalescence of mycelial filaments (in the form of the letter H) which is simply the 

 result of pressure. If it be admitted that the sexual coalescence of cells originated at 

 diflferent times and in different plant-forms, it may also be admitted that in one case 

 a sexual diflferentiation first took place which rendered a coalescence necessary, while in 

 other cases the processes of growth initiated by pressure resulted in the developing of 

 conjugating organs which were sexually differentiated. 



(b) Parthenogenesis '^ is the term used to express the fact that plants which possess 

 normal male organs of fertilisation and in which embryos are developed by the fer- 

 tilisation of the oospheres may occasionally develope embryos from female cells which 

 have not been fertilised, but which are nevertheless capable of complete development. 

 This phenomenon, which is of frequent occurrence in the Animal Kingdom, especially 

 among Insects, has been satisfactorily observed in only a few cases among plants. The 

 doubts as to the parthenogenesis of the Calebogyne ilicifolia which is cultivated in Europe 

 still exists. It appears, however, that Chara crinita is represented in certain places 

 by the female form only, and that nevertheless it bears an enormous number of spores 

 which are capable of germination. The most satisfactory cases of parthenogenesis 

 are those of Saprolegnia ferax and Achlya polyandra ^. Pringsheim has shown that 

 their oospheres are usually fertilised, but that frequently they germinate and develope 



^ [From the fact that, as in Ulothrix for instance, the microzoogonidia come to rest and 

 germinate as well withont as with previous conjugation, it appears probable that the latter is the 

 correct view.] 



^ Braun, Die Parthenogenesis bei Pflanzen, in den Abhandl. der Berl. Akad. 1856.— Pringsheim, 

 Jahrb. f. Wiss. Bot. IX. [See page 593.] 



^ [From de Bary's researches it appears that parthenogenesis is the rule in the Saprolegnia;.] 



