THE ALG^E 



99 



sponges, Hydra, and some Infusoria is due to the presence of 

 minute Protococcoideee. Other forms grow within the intercellu- 

 lar spaces of various aquatic Flowering Plants, while the gonidia of 

 many Lichens are identical with certain species of Protococcoidese. 



Of the simpler Protococcoideae, one of the commonest is Pleurococcus vulgaris 

 (Fig. 68, A), the commonest of the dark-green slimes on bricks, flower-pots, 

 and similar objects. The individual plant is a small globular cell with definite 

 cell-membrane, several chromatophores, and a centrally placed nucleus. The 

 cells multiply rapidly by repeated fission, but no motile cells are produced, and 

 no sexual cells (gametes) are known. Other forms, e.g. Chlorosphsera, give 

 rise to swarm-spores closely resembling the simpler Volvocacese, while a few of 

 them, e.g. Tetraspora, have also simple sexual cells. The cells in Tetraspora 



D 



FIG. 69. Hydrodictyon utriculafum. A, protoplasm of a cell dividing into zoospores. 

 B, two free zoospores. C, zoospores uniting to form the young net. D, a some- 

 what older stage than C. E, a single cell of D, n\ore highly magnified, showing 

 the single equatorial chromatophore with a single pyrenoid and a single nucleus. 

 F, conjugating gametes (X 900). (A, x 600; B, E, X 1000: the others, X about 

 500. F, after KLEBS.) 



and Apiocystis (Fig. 68, B) are imbedded in a gelatinous matrix, which in the 

 former is a flat green thallus closely resembling Ulva, to which probably these 

 forms are related. 



Hydrodictyaceae. The most specialized of the Protococcoidese, which con- 

 sist of cell-families of definite form, are the Hydrodictyacese, in which no cell- 

 division takes place, except when new families are to be formed. The simplest 

 of these (sometimes placed in the Pleurococcacese) are represented by the com- 

 mon genus Scenedesmus (Fig. 68, D), whose cell-families consist of from two to 

 eight spindle-shaped cells, sometimes with long appendages growing from the 

 end cells. In reproduction, each cell divides into from two to eight daughter- 

 cells, which at once arrange themselves in the form of the mature plant. No 

 other form of reproduction is known. 



Hydrodictyon. Hydrodictyon, the Water-net (Fig. 69), is the representative 

 of the family. The fully developed colony has the form of an elongated hollow 

 net, sometimes ten centimetres or more in length. The individual cells of which 

 it is composed finally may reach a length of several millimetres. The cells are 



