BROWN ALGAE 



383 



as well as plurilocular sporangia. Environmental conditions influence the 

 preponderance of one or other of the two alternating phases : thus plants of 

 Ectocarpus in Northern Europe are mainly sporophytic whereas those on the 

 Mediterranean coast are mainly gametophytic 



The Ectocarpale- show various stages in sexual differentiation. Although in 

 Ectocarpus the male and female gametes are morphologically identical, physio- 

 logical differences may be observed Those gametes which are functionally 

 female tend to move more slowly and for a shorter time and to become centres 

 of attraction for the still motile males. In some species the gametes are of 

 unequal size (heterogamous) and in related genera within the Order there may 

 even be an advance to incipient oogamy. In Cutleria (Cutleriales) the two 

 gametes differ obviously in size (Fig. 286). 



Fig. 286. 



On left, Cutleria multifida, showing the smaller, male gametangia (or plurilocular 

 sporangia) and the larger female. Top-left are spermatozoids, top-right are ova. 

 Below are three stages of fertilisation. ( x 500.) (After Reinke.) 



On right, Laminaria digitata. A, Male gametophyte ; a, empty anthendia. 

 B, C, D, Female gametophytes (B is large, C small, while D is reduced to a single 

 oogonium) ; og, oogonium ; 0, ovum. E, young sporophyte, still seated on the 

 empty oogonium. (A x Coo ; B x 292 ; C x 322 ; D x 625 ; E x 322. After 

 H. Kylin.) 



In Dictyota dichotoma (Dictyotales) there is also isomorphic alternation of 

 generations, the sporophytic plant bearing tetraspores. Separate plants in 

 Dictyota dichotoma bear respectively tetraspores, oogonia, and antheridia. 

 The nuclei in the first of these plants are diploid, with 32 chromosomes. The 

 nuclei of the male and female plants are haploid, with only 16 chromosomes. 

 Reduction takes place when the tetraspores are formed, and these on ger- 

 mination give the sexual plants. The analogy of the tetraspores with the spore- 



B.B. 2B 



