The apical growth of Thallus and structure of typical cell in Rhodophyceae!
Methods of growth of thallus:
The growth may be diffused or apical. In the order Bangiales, Porphyra and Asterocystis any cell of the thallus may divide in any direction.
This is called diffused growth. The apical growth is strictly restricted to sub-class Florideae.
Structure of the cell:
The cell wall is two layered. The outer layer consists of pectic substance and layer next to protoplast of cellulose. Sometimes the cell wall contains iodine in it. In majority of forms a thick cuticle like structure of pectic substance is found. In Ceramiaceae the wall becomes stratified.
Pit connections. The members of order Bangiales lack connections. At the point of contact of two cells there is a pit membrane surrounded by a disc of pectic material. Schimitz reported that these are of cytoplasm.
The protoplasm is highly viscous. In order Bangiales there is no central vacuole, but in all Florideae the cytoplasm possesses a conspicuous central vacuole. The cell sap is either alkaline or neutral and rarely acidic.
In lower Rhodophyceae the cells are uninucleate, e.g., Cryptonemiales, Bangiales, etc. In other cases the cells are multinucleate, e g., Ceramiales, Rhodymeniales, etc. Some of the larger multinucleate cells possess 3000 to 4000 nuclei in each of them. The nuclei are with one or more prominent nucleoli. There is well developed chromatin network.
Chromatophores and pigments:
In order Bangiales (lower Rhodophyceae) there is a single stellate chromatophore in each cell. The back cells of Nemalion have ridged chromatophores. In majority of Rhodophyceae the number of chromatophores per cell is more than one. They may be band-like, e.g., in family Ceramiaceae, irregularly lobed or discoidal, e.g., in Polysiphonia.
In each chromatophore there is a centrally placed naked (without starch sheath) pyrenoid.
The pigments of red algae are r-Phycoeythrin (red-water soluble pigments), r-phycocyanin (blue water soluble pigment). Chlorophyll a, Chlorophyll b (in traces only), xanthophylls and carotenes ((J-carotene).
Reserve food products:
The most important food product is floridean starch (polysaccharide) which is found in the form of small grains usually around the nuclei. These grains are not found within the chromatophores. In many Rhodophyceae a soluble sugar floridoside is found. This is galactoside of glycerol. A trehalose polysaccharide water soluble sugar is reported from Lemanea. The fats are found in traces.