228 SEXUAL REPRODUCTION (CONJUGATE) 



(Fig. 122, A and E), the conjugation canals (c.c.) forming loop- 

 like connections between their contiguous ends. Here the threads 

 must be regarded as including cells of both sexes, the zygospores 

 as before being formed either in the conjugation canal or in one 

 of the two cells. The fact that both methods of conjugation may 

 occur simultaneously in the same mass of Spirogyra or Zygnema 

 indicates that sexual differentiation between the filaments is not 

 very profound. 



In all cases the zygospores (Fig. 122, z.) secrete a thick several- 

 layered wall and pass through a prolonged resting period, during 

 which they may. -be distributed in the customary manner by the 

 wind. On germination the membrane usually bursts at two 

 places, the contents growing out in the one direction to produce 

 the new filament, and in the other direction to form the colourless 

 attaching cell. 



Among the Desmids where, except in a few forms, sexual 

 reproduction is rarely observed, fusion takes place between the 

 liberated protoplasts of two individuals, which usually become 

 enveloped in mucilage. The empty halves of their cell- walls 

 are often recognisable near the resulting zygospores (Fig. 114, I), 

 which frequently have elaborately sculptured membranes. In 

 some of the Diatoms a sexual process of an analogous type is 

 encountered. 



The Red Algae possess only motionless reproductive cells, and 

 exhibit a very complex sexual process whose description is 

 beyond the scope of this book. Mention may, however, be made 

 of the copious production of filamentous outgrowths from the 

 female organ after fertilisation, the ends of these threads giving 

 rise to special asexual reproductive cells known as carpospores. 

 The dense clusters thus produced are often conspicuous as minute 

 oval patches of a darker colour (Fig. in, A). The ordinary 

 asexual cells, so-called tetraspores, are produced in fours in small 

 usually spherical sporangia, and are .readily recognised on the 

 thalli under the microscope (Fig. in, C). 



The examples of reproductive processes among the Algae 

 might be multiplied considerably, but sufficient have been 

 described to show the diversity of methods by which the same 

 end, namely the multiplication and perpetuation of the species, 

 is attained. It is the result rather than the means which must 



