The Charophyta are plants containing chlorophyll, living in fresh and brackish water; the stem is jointed, and bears at the nodes whorls of leaves, on which are borne the reproductive organs. The antheridia are spherical in shape and of complex structure, containing numerous biciliate antherozoids. The oogonia are oval in form and contain a single large egg-cell. The Chara-plant is developed from a protonema formed from the germinating oospore. Vegetative reproduction is effected by means of bulbils, accessory shoots, etc.
The Nitelleæ have not been recognised in a fossil condition. The absence or feeble development of a calcareous incrustation renders the genera of this family less likely to be preserved than such a genus as Chara.
Chareae.
Leaves and stems with or without a cortical investment. Fruit with a five-celled corona. The envelope of the ‘fruit’ and other parts of the plant are frequently encrusted with carbonate of lime.
In the genus Chara, the best known member of the family, the plant as a whole resembles in its general habit and external differentiation of parts the higher plants. The stem consists of long internodes separated by short nodes bearing whorls of leaves. Each internode consists of a long cylindrical cell, which becomes enclosed by a cortical sheath composed of rows of cells which have grown upwards and downwards from the peripheral nodal cells. The cortical cells are usually spirally twisted and impart to the stem a characteristic appearance; they are divided by transverse walls into numerous cells some of which occasionally grow out into short processes (fig. 45 c). The leaves repeat on a smaller scale the structural features of the stem, but possess a limited growth, whereas the stem has an unlimited power of growth by means of a large hemispherical apical cell. Branches arise in the axils of the leaves. The plants are either monoecious or dioecious. The oogonium is elliptical in shape, and is borne on a short stalk-cell, it contains a single oosphere. The wall of the oogonium is formed of five spirally twisted cells which have grown over it from the five peripheral cells of a leaf-node. The tips of the investing cells project at the apex in the form of a terminal crown or corona (fig. 45, E, c). The antheridia have a complex structure, and produce a very large number of motile antherozoids.
Fig. 45. A and B. Chara Knowltoni Sew. From a section in the British Museum. C. Stem of Chara foetida A. Br. in transverse section (after Migula. × 18). D. Interior of oogonium of C. foetida. E. Oogonium of C. foetida (D and E after Migula. × 50).
After fertilisation, the egg-cell becomes surrounded by a membrane, at first colourless, but afterwards yellow or brown. The inner cell-walls of the cells surrounding the oospore become thicker and darker in colour; the outer walls remain thin and eventually fall away. The lateral walls may or may not become thickened. In most of the Chareae a calcareous deposit is formed between the hard shell and the outer walls of the cells enveloping the oospore. This calcareous shell is developed subsequently to the thickening and hardening of the inner walls of the fruit-case. The cells of the corona and stalk do not become calcareous. In the fossil Charas, it is this calcareous shell that is preserved. In the members of the Chareae the stems are usually encrusted with carbonate of lime, and thus have a much better chance of preservation than the slightly calcareous Nitelleæ.
Chara.
The generic characters have already been described in the brief account of the family Chareae.