The Algae - Phaepophycae - Ectocarpales - Ectocarpus

Phylum Thallophyta - The Algae - Phaepophycae - Ectocarpales - Ectocarpus
The Ectocarpales are a small group of relatively simple Phaeophyceae in which the thallus is filamentous in its fundamental structure, though very varied in form. In the simplest cases the filaments are entirely free, but in some other forms the lower parts of the filaments are variously aggregated together to form more or less solid bodies, the ends alone remaining free.
The vegetative cells in Ectocarpales are shortly cylindrical, containing irregularly shaped chromoplasts with pyrenoids. There is no special apical cell. Growth takes place by the division of an intermediate group of cells in the filament, which form a meristematic zone. Such a method of growth, which is neither apical nor basal, is called intercalary.
There are two characteristic reproductive structures. Firstly, unilocular sporangia, which consist of single enlarged cells, the contents of which divide up, without the formation of any cell walls, to liberate a large number of motile cells which are pear-shaped and have two flagella, one directed forwards and the other backwards. Secondly, gametangia (plurilocular sporangia), derived from the cells of a short side branch. These divide up by transverse and longitudinal walls into a large number of small cubical cells, each of which produces a biflagellated motile gamete, similar in structure to the products of the unilocular sporangia.
Meiosis takes place at the first nuclear division in the unilocular sporangium, so that the spores from this type are definitely monoploid. In general, they function as asexual zoospores, giving rise to haploid plants. The plurilocular sporangia, on the other hand, are generally regarded as gametangia, the cells from which fuse in pairs, and from the zygote thus formed diploid plants arise. Variations of this behaviour are, however, known, as explained below.
One example of this order is Ectocarpus, but as there is considerable variation in details between species, the description must necessarily be a general one.
Ectocarpus
Species of the genus Ectocarpus are very common in sea water around the British coast. Some are attached to rocks and stones, but many grow attached to other Algae, and are therefore said to live as epiphytes. '''Te must dis­tinguish between an epiphyte, in which the plant obtains no nourishment from the organism on which it lives, and a parasite in which the reverse is the case; for the parasites live at the expense of the second plant, which is then termed the host plant.
The thallus of Ectocarpus is generally filamentous and repeatedly branched. It is made up of two parts, a creeping portion which covers the substratum and serves as the holdfast, and a number of upright branches which arise from it. These upright branches  generally consist of a single row of cells, though in some species longitudinal cell divisions occur. The cells are small and rectangular, each con­taining a single nucleus and a number of small spherical bodies containing the brown pigment, which are called chromatophores or chromoplasts. Pyrenoids are present in the chromo­plasts. This is interesting because these bodies, which are characteristic of the Chlorophyceae, are only found in the most primitive members of the Red and Brown Algae.
Growth takes place by the division of a series of small cells situated near the base of the vertical branches. These cells are generally small and continue throughout the life of the branch to cut off additional cells from their upper sur­faces. The cells of the more apical part of the branch do not divide and are more elongated than those of the actively divid­ing or meristematic region. This is therefore an example of intercalary growth .
Asexual Reproduction in Ectocarpus
The asexual reproductive organs consist of unilocular zoosporangia which are developed on the side branches (Fig. 94). They originate as simple globular or pear-shaped cells which become densely filled with protoplasm. The original nucleus divides first meiotically and then repeatedly by mitosis. Around each daughter nucleus a small quantity of protoplasm collects to form a zoospore. It consists of a small, rather elongated cell, and contains besides the nucleus a single brown plastid. Two flagella are developed at one side of the zoospore, one pointing forwards and the other backwards. On liberation the zoospore swims away, eventually settles down and grows into a monoploid plant.
Sexual Reproduction in Ectocarpus
The sex organs are borne on monoploid plants and consist of gametangia , which develop laterally, either sessile on a branch or at the end of a short row of sterile cells.* Each gametangium is composed of a number of chambers, separated from one another by transverse and longitudinal walls. In each of these com­partments one, or sometimes two, gametes are formed.
They do not differ mor­phologically from the asexual zoospores, but on liberation they fuse in pairs. In some species, e.g., E. secundus, there is a difference in the size of the gametes, and in such cases it is seen that the smaller ones cluster around the larger ones. It is con­cluded that the larger repre­sent the female gametes, while the smaller are regarded as the males. After conjugation the zygote develops into a diploid plant, which eventu­ally produces sporangia.
Sexual plants are as a rule dioecious, gametes from one plant behaving for the most part consistently either as males or as females. There are, however, exceptions, cases of irregular behaviour, which led Hartmann to the doctrine Fr of relative sexuality, which regards sex as a quantitative character, which may be either <:trnno- nr weak. so that a  given cell may behave as either male or female according to the strength of sex in the cell with which it pairs.
As mentioned above under the heading of Ectocarpales, certain variations in the life history of Ectocarpus exist, which have given rise to considerable controversy in regard to its methods of reproduction. The principal point to be noticed is that both monoploid and diploid plants occur, which are morphologically indistinguishable. There is evidence that in some species they may be geographically separated. Monoploid plants appear to form only gametangia, which produce gametes that fuse in pairs. They may therefore be considered as true gametophytes. Diploid plants, on the other hand, may produce both gametangia and zoosporangia. In this case the products of the gametangia are diploid and germinate without conjugation like zoospores. The products of the unilocular sporangia are, however, monoploid.  Most observers agree that they too germinate without fusion and regard them as true zoospores. It has been claimed, however, that they may fuse in pairs, producing diploid plants, but the actual develop­ment of such apparent conjugations has never been observed. If it occurs we have here a complete inversion of the normal reproductive order
The occurrence of con­jugation between the motile 2 cells produced in sporangia has been interpreted as im­plying, in these cases, the complete suppression of the gametophyte stage, the pro­ducts of the sporangia be­having directly as gametes instead of germinating to produce another vegetative generation. This special case in Ectocarpus would therefore be comparable to that in the normal life-history of Fucus.

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