. Botany of the living plant. Botany; Plants. 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
. Botany of the living plant. Botany; Plants. 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-
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