CHLOROPHYCEAE 9 



The South African Chlorophyceae do not differ materially from the Green 

 Algae met with in other parts of the world, these plants being as a rule 

 cosmopolitan in their distribution. One of the very simplest forms, ranking 

 indeed amongst the most primitive organisms in the plant kingdom, is that 

 which causes, in some parts of the world, the phenomenon known as " red 

 rain." This is the genus Sphaerella (Haematococcus). It is a very minute 

 unicellular alga, which in its ordinary condition is freely motile by means 

 of two fine cilia at the forward end of the cell. These propel the plant in 

 somewhat the same manner as oars propel a boat. 



As shown in Plate 2, Fig. A, the cell-wall of this plant stands out from 

 the protoplasm and the latter contains a nucleus and a single basin-shaped 

 chloroplast. Sphaerella readily passes, however, into a dormant condition, 

 in which its colour becomes bright red, and it is this stage which causes the 

 " red rain." This is called the palmella condition (Fig. A, 2). 



Two methods of reproduction are met with in the genus : 



(a) Formation of two or four smaller motile cells, destitute of a cell- 

 wall, by division of the contents of a single mother-cell. These are called 

 zoospores ; after a time they form a cell-wall and acquire the characters 

 of the parent plant. 



(b) Formation of several still smaller motile, naked cells, called gametes, 

 within a single mother-cell. These, on escaping, fuse together in pairs to 

 form cells which remain round and motionless for a time (the zygotes), 

 but resume the motile condition subsequently. 



As an example of one of the larger green algae, Enteromorpha may 

 be mentioned # . The prevalent Cape species, E. intestinalis, is usually found 

 growing in dense tufts, on the rocks about high water, much like a tuft of 

 grass. The plants are more or less cylindrical, but narrower towards the 

 base and somewhat constricted at fairly regular intervals. The cylinder is 

 hollow and bounded by a single layer of small regularly shaped cells. 

 Near the base the axis gives off a number of similar branches. (See 

 Plate 2, Fig. E.) Reproduction may be either by means of minute, motile 

 sexual cells (gametes) , which fuse together in pairs to form a zygote, the 

 zygote subsequently growing into a new plant ; or, by means of small zoospores, 

 which, after coming to rest, at once grow to form new plants, attached to the 

 rocks by means of a slender, branched, root-like basal part. 



Although the large majority of the Green Algae are really bright green 

 in colour, yet there are isolated instances of plants with quite different colouring, 

 which, however, are obviously so closely related to green forms that it would 

 be inconvenient to separate them from the family in which they are included, 

 on the ground of colour alone. An example of such an alga is figured in 

 Plate 2, Fig. N. This is named Trentepohlia and is frequently found grow- 

 ing on the under side of overhanging rocks in sheltered situations of the 



* This alga is one of the principal constituents of the so-called " grass " which 

 fouls the bottom of ships, hence its universal distribution. 



m. 2 



