Gamogenesis 



135 



N 



SEXUAL REPRODUCTION. From the point of view of sexuality the 

 Chlorophycese present a most interesting, and almost unique, series of forms. 

 Within the group are to be found all conditions from a simple gamogenesis 

 to the union of highly specialized gametes. 

 Every degree of differentiation of the gametes 

 is met with from those which are externally 

 alike (isogametes) to those which are to a 

 lesser or greater degree unlike (heterogametes). 

 The union of the gametes results in a zygote, 

 which may be either a zygospore or an oospore. 



The term 'sexual' is often used to embrace all 

 forms of gamogenesis, and is in this sense firmly es- 

 tablished in botanical literature. It would, however, 

 be more scientifically accurate to distinguish between 

 gamogenesis (or the mere fusion of gametes) and sexual 

 reproduction in the narrower sense (which should be 

 restricted to those cases where there is a fusion of 

 clearly differentiated < and $ gametes). From this 

 point of view, therefore, gamogenesis although includ- 

 ing sexual reproduction is not identical with it. It 

 must be remarked, however, that the gradation is so 

 fine, especially in the Chlorophycese, that the distinc- 

 tion is scarcely worth making. Sometimes, as in 

 many of the Zygnemaceae, the gametes are morpho- 

 logically indistinguishable but physiologically differ- 

 entiated, and in these cases there is often a morpho- 

 logical differentiation of the gametangia (consult fig. 

 92.4). 



The zygospore is formed by the union of 

 like (or nearly like) gametes, such as takes 

 place in many of the Protococcales, Ulotri- 

 chales, Desmidiacea?, etc., and is the result 

 of a gamogenesis in which there are no 

 external sexual differences between the 

 gametes themselves. The gametes arise in 

 gametangia. In the unicellular forms the 

 cell itself becomes the gametangium (Des- 

 midiacea?, many Protococcales, etc.), and in 

 the multicellular forms the ordinary vege- 

 tative cells become the gametangia with or 

 without some external change of form (Zyg- 

 nemaceae, Ulotrichales, etc.). In the Conjugates the gametangium gives 

 origin to only one large non-ciliated aplanogamete, but in other groups it is 

 more usual for several ciliated planogametes to be formed in one gametangium. 



Fig. 92. Various forms of little- 

 differentiated gametes. A, Spira- 

 lly ra tenuissima (Hass.) Kilt/, 

 showing aplanogarnetes and 

 slightly differentiated gametan- 

 gia. B H, Planogametes. B, 

 Isogamete of Trentepohlia Blei- 

 schii (after Karsten) ; C, fusion 

 of heterogametes of Pandorina 

 Morum (Mull.) Bory; B -F, es- 

 cape of isogametes from gamet- 

 angium, and fusion (E) to form 

 zygote (F), of Monostroma mem- 

 branacea W. & G. S. West; G 

 and H, gamete (G) and gamo- 

 genesis (H) of Chlamydomonas 

 media (after Klebs) ; gt, gamet- 

 angium. 



