THE ALGAE 



"3 



Sexual Reproduction 



The sex organs are borne on monoploid plants and consist of gametangia 

 (Fig. 95), which develop laterally, either sessile on a branch or at the end 

 of a short row of sterile cells.* Each gametangium is composed of a number 

 of chambers, separated from one another by transverse and longitudinal 

 walls. In each of these com- 

 partments one, or sometimes 

 two, gametes are formed. 



They do not differ mor- 

 phologically from the asexual 

 zoospores, but on liberation 

 they fuse in pairs. In some 

 species, e.g., E. seciindus, there 

 is a difference in the size of 

 the gametes, and 



ZOOSPOR 

 ANGIUM 



m 



ZONE OF 

 such INTERCALARY 

 cases it is seen that the GROWTH 



smaller ones cluster around 

 the larger ones. It is con- 

 cluded that the larger repre- 

 sent the female gametes, while GAMETANGIUM 

 the smaller are regarded as 

 the males. After conjugation 

 the zygote develops into a 

 diploid plant, which eventu- 

 ally produces sporangia. 



Sexual plants are as a rule 

 dioecious, gametes from one 

 plant behaving for the most 

 part consistently either as 

 males or as females. There 

 are, however, exceptions, cases /. 



of irregular behaviour, which 

 led Hartmann to the doctrine 

 of relative sexuality, which 

 regards sex as a quantitative 

 character, which may be either 



strong or weak, so that a given cell may behave as either male or female 

 according to the strength of sex in the cell with which it pairs. 



As mentioned above under the heading of Ectocarpales, certain variations 

 in the life history of Ectocarpiis exist, which have given rise to considerable 

 controversy in regard to its methods of reproduction. The principal point 

 to be noticed is that both monoploid and diploid plants occur, which are 

 morphologically indistinguishable. There is evidence that in some species 

 they may be geographicallv separated. Monoploid plants appear to form 



* In Pylaiella, a very common and closely related Alga, the gametangia are intercalary. 



5 



Fig. 93. — Ectocarpiis sp. A, Branching filaments 

 showing intercalary growth and reproductive 

 organs. B, Young zoosporangium. C, Zoospore. 

 D, Gametangium. E, Conjugation 01 gametes. 



