The Algae - Phaepophycae - Dictyotales - Dictyota dichotoma

Phylum Thallophyta - The Algae - Phaepophycae - Dictyotales  - Dictyota dichotoma
These Algae are Phaeophyceae in which a definite alternation of genera­tions occurs and in \yhich the sporophytic and gametophytic plants are morphologically indistinguishable. Sexual reproduction is oogamous, while the organ of asexual reproduction is a non-motile spore termed a tetraspore, so called because only four spores are produced in each sporangium.
This is a small order with only a few genera, and \ye shall consider Dictyota dichotoma as our example.
Dictyota dichotoma
This Alga is common around the coast of Britain, growing in pools between tide-marks. The strap-like thallus is 10 to 20 em. long and consists of rectangular cells arranged in a single layer, with a superficial layer of smaller cells on each side of the thallus. Small tufts of hairs develop from scattered groups of these surface cells. The thallus is attached to the rock by a basal holdfast.
The thallus branches repeatedly, each division giving rise to two equal branches. Such a type of branching is termed dichotomous. Growth is by means of one large apical cell on each branch, which divides vertically into two equal halves when branching is about to take place. The male and female organs are borne on separate plants, and the asexual reproductive organs are produced on different plants from the sexual ones; but, as far as their general appearance is concerned, all three kinds of plants are exactly the same.
ASEXUAL REPRODUCTION 
The sporangium arises from a superficial cell of the thallus, which grows into a small spherical projection from the surface. Its nucleus enlarges and then divides meiotically, so that four monoploid nuclei are produced and the cytoplasm divides into four portions. No separating walls are formed, but the four monoploid spores are eventually liberated by a breakdown of the wall of the sporangium. These spores have no flagella.
The term tetraspore, which is applied to them, is used also for the asexual spores in the Rhodophyceae which are also formed in fours, but the number is not constant in the Dictyotaceae, some genera producing eight in each sporangium. The tetrasporangia are, of course, equivalent to the sporangia of Ectocarpus.
On germination a tetraspore produces a fresh Dictyota plant, resembling in appearance the parent from which it was developed, but bearing gametes.
SEXUAL REPRODUCTION 
The sex organs are antheridia and oogonia. They are arranged in groups or sori on the surface of the thallus on separate plants. Each antheridium arises from a superficial cell of the thallus. This cuts off a lower stalk cell and an upper or antheridial cell which enlarges and divides, like a gametangium of Ectocmpus, into a large number of parts, so that, at maturity, some 1,500 antherozoids are liberated. As there are between 100 and 300 antheridia in each sorus, and as the number of sari on a full-grown plant may be well over 3,000, there is a fortnightly production of something like 500 million antherozoids from each mature plant, which means that there are on the average about 6,000 male gametes to each female. These antherozoids are pear-shaped and very small, and each is provided with two lateral flagella, the one which is directed backwards being much reduced in length. Each antheridial sorus is surrounded by two or three rings of sterile cells, called the paraphyses.
The oogonium also develops from a superficial cell of the female thallus. This cell divides into two, the lower forming a stalk cell, and the upper the oogonium. Inside the oogonium a single oosphere is formed, which is liberated as a naked mass of protoplasm by the breakdown of the oogonial wall. Each female sorus contains twenty-five to fifty oogonia which are not surrounded by paraphyses.
Both kinds of gametes are set free in large quantities at intervals of fourteen days, shortly after the highest spring tides. This periodicity is hereditary and is maintained even under culture in aquaria. Fertilization occurs in the sea water and is effected by the movement of the antherozoids in large numbers to the oosphere, followed by the penetration of the oosphere by one of the antherozoids. Even if an oosphere fails to be fertilized it is capable of development to a limited extent.
Under normal circumstances the zygote germinates at once to produce a fresh diploid plant, like the parents in appearance, but on ,vhich eventually tetrasporangia will be produced.
It will be seen, then, that Dictyota exhibits a typical alternation of genera­tions between a diploid tetrasporic plant on the one hand, which is regarded as the sporophyte, and two haploid sexual plants, the male and the female, which constitute the gametophytes.

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