In this article we will discuss about the thallus and cell structure of chara with the help of suitable diagrams.
Thallus Structure of Chara:
The thallus of Chara is branched, multicellular and macroscopic. The thallus is normally 20-30 cm. in height but often may be up to 90 cm to l m. Some species like C. hatei are small and may be 2-3 cm. long. The plants in appearance resemble Equisetum hence Chara is commonly called as aquatic horsetail. The thallus is mainly differentiated into rhizoids and main axis (Fig. 1).
The rhizoids are white, thread like, multicellular, uniseriate and branched structures. The rhizoids arise from rhizoidal plates which are formed at the base of main axis or from peripheral cells of lower nodes. The rhizoids are characterized by presence of oblique septa (Fig. 2).
The tips of rhizoids possess minute solid particles which function as statoliths. The rhizoids show apical growth. Rhizoids help in attachment of plant to substratum i.e., mud or sand, in absorption of minerals and in vegetative multiplication of plants by forming bulbils and secondary protonema.
2. Main Axis:
The main axis is erect, long, branched and differentiated into nodes and internodes. The internode consists of single, much elongated or oblong cell. The inter-nodal cells in some species may be surrounded by one celled thick layer called cortex and such species are called as corticate species. T
he species in which cortical layer is absent are called ecorticate species (Fig. 3 A, B). The node consists of a pair of central small cells surrounded by 6-20 peripheral cells (Fig. 3 C). The central cells and peripheral cells arise from a single nodal initial cell.
On nodes develop these following four types of appendages:
(i) Branches of limited growth.
The branches of limited growth arise in whorls of 6-20 from peripheral cells of the nodes of main axis or on branches of unlimited growth. These are also called branchlets, branches of first order, primary laterals or leaves. These branches stop to grow after forming 5-15 nodes and hence are called branches of limited growth. The stipulodes and reproductive structures are formed on the node of these branches.
(ii) Branches of unlimited growth:
The branches – of unlimited growth arise from the axils of the branches of limited growth hence these are also called auxiliary branches or long laterals. These are differentiated into nodes and internodes. At nodes they bear primary laterals and these branches look like the main axis. Their growth is also unlimited like main axis.
The basal node of the branches of limited growth develops short, oval, pointed single cell outgrowths called stipulodes. In most of the species of Chara e.g., C. burmanica, the number of stipulodes at each node is twice the number of primary laterals, such species are called as bi-stipulate.
In some species of Chara e.g., C. nuda and C. braunii, the number of stipulodes at each node, is equal to number of primary laterals at that node, such species are called unitipulate. When stipulodes are present in one whorl at each node the species are called as haplostephanous and with two whorls on each node are called diplostephanous (Fig. 4 A, B).
Many species of Chara e.g., C. aspera, C. inferma have inter-nodal cells of main axis en-sheathed by cortex cells. Such species are called corticated species.
The cortex consists of vertically elongated narrow cells. The internode up to half of its length by corticating filaments developed from upper node called descending the lower half of internode is covered by filaments developed from lower node called filaments. The ascending and descending filaments meet at the middle of internode. T species without cortex e.g., C. corallina are called ecorticated species.
Cell Structure of Chara:
The main axis of Chara consists of mainly two types of cells:
(i) Nodal cells
(ii) Inter-nodal cells.
(i) Nodal Cells:
The nodal cells are smaller in size and isodiametric. The cells are dense cytoplasmic, uninucleate with few small ellipsoidal chloroplasts. The central vacuole is not developed instead many small vacuoles may be present. The cytoplasm can be differentiated in outer exoplasm and inner endoplasm (Fig. 5 A).
(ii) Inter-Nodal Cells:
The inter-nodal cells are much elongated. The cytoplasm is present around a large central vacuole. The cells are multinucleate and contain many discoid chloroplasts. The cytoplasm is also differentiated into outer exoplasm and inner endoplasm. The endoplasm shows streaming movements (Fig. 5 B). The cell walls between the nodal cell and inter-nodal cells are porous to help in cytoplasmic continuity between cells.