100 OUTLINES OF EVOLUTIONARY BIOLOGY 



male and female plants. In both cases certain branches (/), 

 usually described as fertile, are characterized by the presence of 

 numerous minute spherical pits, opening on to the surface by narrow 

 mouths. These pits or conceptacles, one of which is -represented 

 in vertical section in Fig. 46, contain the sexual organs, male 

 antheridia or female oogonia, as the case may be, intermingled 

 with hair-like structures known as paraphyses. The antheridia 

 (Fig. 47, a) are attached to the branching paraphyses in the male 



a 



FIG. 47. Fucus vesiculosus. 



a, branching paraphysis from male conceptacle, bearing antheridia ; b, an oogoiiium 

 surrounded by unbranched paraphyses and with its contents divided into eight ova ; 

 c, a discharged ovum surrounded by spermatozoa, one of which will fertilize it ; d, a 

 developing embryo ; all x 160. (From Vines' " Botany," after Thuret.) 



conceptacles in the form of small sacs in which the male gametes 

 (spermatozoa) are produced. These are minute, nucleated, pear- 

 shaped cells, each with two flagella ; they are produced in large 

 numbers in each antheridium and set free by rupture of the wall 

 of the latter to make their way out of the opening of the 

 conceptacle by their own activity. 



The oogonia are oval sacs, a good deal larger than the antheridia, 

 and occur amongst the hair-like paraphyses in the female con- 

 ceptacles (Fig. 46). In each oogonium (Fig. 47, 1) eight female 

 gametes (egg cells, ova or oospheres) are formed by cell-division. 

 These are spherical, nucleated cells, very much larger than the 



