Cosmarium: Vegetative Body and Reproduction | Algae | Botany

In this article we will discuss about the vegetative body and reproduction of cosmarium.

Vegetative Body of Cosmarium:

Cosmarium is a unicellular genus which may be easily recog­nized in having small compressed cells, each being divided by a conspicuous median constriction (sinus) into two distinct symmetri­cal halves (semi-cells), and united together by a comparatively narrow connecting zone (isthmus). Its species are distinguished partly from such characters as cell shape, ornamentation of the wall, etc.

Each cell should be examined in front, side and vertical views. In front view the semi-cells may be circular, elliptical, reniform or sub-quadrangular in shape, whereas in vertical view they are usually elliptical in outline. In side view the semi-cells, in most cases, appear more or less circular in outline.

The cell wall in cosmarium is never spiny, and it consists of three concentric layers:

(1) An innermost thin cellulose layer,

(2) A somewhat thicker median layer consisting of cellulose and pectic compounds, and

(3) An outer thick or thin layer of gelatinous sheath made up of pectose.

Excepting at the isthmus region the inner two layers are perforated by vertical pores, being usually arranged in a definite pattern and are distributed all over the semi-cells, or may be localized. The outer wall is usually smooth, but may sometimes appear some what granular.

The protoplast contains a single nucleus which always lies at the isthmus and is sometimes connected with the string-like pro­jections of the chloroplast. Each nucleus contains a conspicuous nucleolus and well-defined nuclear reticulum.

Usually, there is only one chloroplast in each semi-cell, but sometimes two or four chloroplasts may be present. Each chloroplast contains a single pyrenoid and is provided with four to several ra­diating plate-like extensions.

The cells of cosmarium exhibit movement which consists in a series of jerks. This is due to the secretion of gelatinous material through the vertical pores.

Reproduction in Cosmarium:

Cosmarium reproduces both by vegetative and sexual methods. The vegetative method of reproduction takes place by the method of cell division and two daughter cells are formed from the parent cell. During the process the nucleus first divides into two daughter nuclei and the isthmus slightly elongates.

A transverse wall is formed across the isthmus in such a way that each semi-cell receives one daughter nucleus. The portion of the isthmus, at­tached to each semi-cell, now enlarges and forms a new semi-cell, and in this way two daughter cells are formed. The daughter cells remain attached to each other for some time but finally separate.

Thus, it is evident that of the two semi-cells, one semi-cell is younger than the other, which belongs to the parent cell. The new­ly formed semi-cell receives a chloroplast with a pyrenoid by the division of the pre-existing one of the other semi-cell. Sometimes pyrenoid may be formed de novo in the newly formed chloroplasts.

Sexual reproduction is isogamous and consists in the union of two identical gametes, produced usually by two mature individuals.

Conjugation may also take place between two newly formed sister cells. During conjugation, usually two mature cells come to lie in contact with each other and become surrounded by a common ge­latinous sheath secreted for the purpose. The protoplast of each cell functions as a gamete.

Each cell then breaks at the isthmus and the gamete escapes. The two gametes, thereupon, fuse outside and after secreting a thick wall around it form a somewhat globose or angular zygospore. Its wall may be smooth, papillate or spiny. The zygospore, after a considerable period of rest, germinates.

The fu­sion nucleus undergoes a reduction division forming four haploid nuclei, of which two degenerate and the remaining two are functional. The protoplast then divides into two equal halves, each receiving one functional nucleus and a chloroplast. Each daughter protoplast finally develops into a vegetative cell, which is liberated by the rupture of the zygote wall.

When conjugation fails, azygospores or parthenospores may be formed. The parthenospore, during germination, contains three degenerating nuclei and one functional nucleus, which, by division, gives rise to the nuclei of the two resulting vegetative cells.