The family consists of multicellular plants, whose cells are firmly united together to form a thallus; this, in the simplest cases, may be a branched filament of cells (Ectocarpus), or, in the highest, may resemble a stem with leaves (Laminariaceæ), while all transitional forms may be found between these two. The thallus grows by intercalary divisions (e.g. Ectocarpus), or by an apical cell (e.g. Sphacelaria); pseudo-parenchymatous tissue may sometimes be formed by cells, which were originally distinct, becoming united together. The size of the thallus varies; in some species it is quite small—almost microscopical,—while in the largest it is many metres in length.
The vegetative cells in the lower forms are nearly uniform, but in those which are more highly developed (Laminariaceæ and Fucaceæ), they are sometimes so highly differentiated that mechanical, assimilating, storing and conducting systems may be found; the last named systems are formed of long cells with perforated, transverse walls, which bear a strong resemblance to the sieve-tubes in the higher plants.
Fig. 65.—Swarmspore of Cutleria multifida.
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 (?).
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.
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.