The colouring matter in the living cells (“phæophyl”) contains chlorophyll; but this is concealed by a brown (“phycophæin”), and a yellow (“phycoxanthin”) colouring material, and hence all these Algæ are a lighter or darker _yellow-brown_. Starch is not formed. Asexual reproduction takes place, (1) by zoospores which arise in unilocular zoosporangia, and are monosymmetric, with two cilia attached laterally at the base of the colourless anterior end (Fig. 65), the longer one being directed forwards and the shorter backwards; or (2) by aplanospores (?). [Illustration: FIG. 66.--_Ectocarpus siliculosus_. _I a-f_ A female gamete in the various stages of coming to rest. _II_ A motionless female gamete surrounded by male gametes. _III a-e_ Stages in the coalescence of male and female gametes.] [Illustration: FIG. 67.--_Zanardinia collaris_. _A_ Male gametangia (the smaller celled) and female gametangia (the larger celled). _C_ Female gamete. _D_ Male gamete. _B_, _E_ Fertilisation. _F_ Zygote. _G_ Germinating zygote.] Sexual reproduction has only been discovered in a few cases, and takes place by means of gametes (oogamous fertilisation perhaps occurs in the Tilopteridæ). The gametes have the same structure as the zoospores, and arise in multilocular gametangia; these, like the zoosporangia, are outgrowths from the external surface, or arise as modifications from it. The conjugating gametes may be similar (_e.g. Ectocarpus pusillus_), or there may be a more or less pronounced difference of sex, an indication of which is found in _Ectocarpus siliculosus_ (Fig. 66). When the gametes in this species have swarmed for a time, some, which are generally larger, are seen to attach themselves by one of the cilia, which by degrees is shortened to form a kind of stalk (compare the upper gamete in Fig. 66 _II_); these are the female gametes, which now become surrounded by a number of males endeavouring to conjugate with them, but only one succeeds in effecting fertilisation. The protoplasm of the two gametes coalesces (Fig. 66 _III_), and a zygote (_e_) is formed. The male gametes which do not conjugate may germinate, but the plants derived from them are much weaker than those produced by the zygotes. Strongly pronounced sexual differences are found in the Cutleriaceæ, in which order the male and female gametes arise in separate gametangia (Fig. 67 _A_). The male gametes (Fig. 67 _D_) are much smaller than the female gamete (Fig. 67 _C_); the latter, after swarming for a short time, withdraws the cilia, and is then ready to become fertilised (Fig. 67 _B_, _E_), thus we have here a distinct transition to the oogamous fertilisation which is found in the Fucaceæ. Alternation of generations is rarely found. 1. Sub-Family. =Zoogonicæ.= Reproduction by means of gametes and zoospores.