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Phylum Thallophyta - The Algae - Chlorophycae - Siphonales - Bryopsis plumosa
The Siphonales are Chlorophyceae in which the thallus consists of a single multinucleate cell or coenocyte, which often grows to form a structure of macroscopic size. Asexual reproduction occurs in a few genera by the formation of zoospores. The sexual reproduction in many of the marine genera is by the fusion of unlike motile gametes; that in the fresh-water genera IS oogamous.
Some of these coenocytic Green Algae show great complexity. For example, in the genus Caulerpa, there is a complex development of coenocyte branches to form a thallus, which may be as much as 10 em. in height, with a definite external form. The thallus in Caulerpa is especially remarkable, since the branches of the single coenocyte build up thalli that simulate in appearance the differentiation into roots, stems and leaves found in the higher land plants.
Bryopsis plumosa
The genus Bryopsis is distributed mainly in tropical seas, but one species, B.plumosa, is commonly found in spring and early summer on the British coasts. The thallus consists of a single coenocyte, but this enormous ce shows differentiation into a main axis, from which arise, towards the up end, two rows of lateral branches or pinnae. From the lower end of the axis there is formed a little-branched horizontal rhizome anchored by rhizoids. This rhizome may produce numerous upright axes, so that each plant actually consists of a little tuft of vertically growing filaments. The pinnae vary in length, those nearest the base being the longest and decreasin regularly towards the apex. Each pinna is an elongated sac, and there is a constriction at the base where it joins the main axis. No true septa are formed in the coenocyte prior to the reproductive phase and the entire plant has one continuous vacuole lined by cytoplasm and containing numerous minute, round chloroplasts and nuclei.
Vegetative propagation may be effected by the detachment of pinnae which become plugged at the point of abstriction. These are able to develop new rhizoids and grow into fresh plants. It is interesting to note that in conditions of dull light or when plants are placed upside down the apices of the pinnae develop rhizoids.
The only known method of reproduction is by means of gametes. There is no asexual method of reproduction.
SEXUAL REPRODUCTION
The gametes are not alike and are usually produced on separate plants.
The first stage consists of the cutting off of a pinna from the main axis by means of a septum which arises as a ring-like thickening. The whole of the pinna thus forms a 'gametangium whose protoplasmic contents increase and the chloroplasts multiply by division. In the male gametangium the pyrenoids disappear from the chloroplasts, but they remain in those of the female gametangium. Later the contents divide up by simultaneous cleavages to form gametes, which are liberated by the gelatinization of the apex of the pinna. The gametes are pyriform with two equal, apical flagella. The female gamete is about three times as large as the male and is provided with a deep-green chloroplast. In the male the cWoroplast is yellowish in colour and probably not functional. Gametes fuse in pairs and produce a zygote which may, for a time, retain all four flagella. Later it rounds off and germinates directly to produce a new plant. The plants are diploid and reduction division occurs in the formation of the gametes
The Siphonales are Chlorophyceae in which the thallus consists of a single multinucleate cell or coenocyte, which often grows to form a structure of macroscopic size. Asexual reproduction occurs in a few genera by the formation of zoospores. The sexual reproduction in many of the marine genera is by the fusion of unlike motile gametes; that in the fresh-water genera IS oogamous.
Some of these coenocytic Green Algae show great complexity. For example, in the genus Caulerpa, there is a complex development of coenocyte branches to form a thallus, which may be as much as 10 em. in height, with a definite external form. The thallus in Caulerpa is especially remarkable, since the branches of the single coenocyte build up thalli that simulate in appearance the differentiation into roots, stems and leaves found in the higher land plants.
Bryopsis plumosa
The genus Bryopsis is distributed mainly in tropical seas, but one species, B.plumosa, is commonly found in spring and early summer on the British coasts. The thallus consists of a single coenocyte, but this enormous ce shows differentiation into a main axis, from which arise, towards the up end, two rows of lateral branches or pinnae. From the lower end of the axis there is formed a little-branched horizontal rhizome anchored by rhizoids. This rhizome may produce numerous upright axes, so that each plant actually consists of a little tuft of vertically growing filaments. The pinnae vary in length, those nearest the base being the longest and decreasin regularly towards the apex. Each pinna is an elongated sac, and there is a constriction at the base where it joins the main axis. No true septa are formed in the coenocyte prior to the reproductive phase and the entire plant has one continuous vacuole lined by cytoplasm and containing numerous minute, round chloroplasts and nuclei.
Vegetative propagation may be effected by the detachment of pinnae which become plugged at the point of abstriction. These are able to develop new rhizoids and grow into fresh plants. It is interesting to note that in conditions of dull light or when plants are placed upside down the apices of the pinnae develop rhizoids.
The only known method of reproduction is by means of gametes. There is no asexual method of reproduction.
SEXUAL REPRODUCTION
The gametes are not alike and are usually produced on separate plants.
The first stage consists of the cutting off of a pinna from the main axis by means of a septum which arises as a ring-like thickening. The whole of the pinna thus forms a 'gametangium whose protoplasmic contents increase and the chloroplasts multiply by division. In the male gametangium the pyrenoids disappear from the chloroplasts, but they remain in those of the female gametangium. Later the contents divide up by simultaneous cleavages to form gametes, which are liberated by the gelatinization of the apex of the pinna. The gametes are pyriform with two equal, apical flagella. The female gamete is about three times as large as the male and is provided with a deep-green chloroplast. In the male the cWoroplast is yellowish in colour and probably not functional. Gametes fuse in pairs and produce a zygote which may, for a time, retain all four flagella. Later it rounds off and germinates directly to produce a new plant. The plants are diploid and reduction division occurs in the formation of the gametes
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