Palmellacece 239 



Family 8. PALMELLACEiE. 



The Palmellacese is one of the most primitive families of green 

 Algae, primarily distinguished from the other groups of the 

 Protococcoideae by the indefinite colonies of cells enveloped in a 

 conspicuous mass of mucilage. In the ordinary vegetative con- 

 dition these plants present the appearance of a group of more 

 or less irregularly disposed cells embedded in a copious mass of 

 jelly, which is either structureless or differentiated into concentric 

 envelopes. The colonies are either microscopic or macroscopic, 

 and sometimes reach a length of several centimetres. 



The cells are globose or ellipsoid, of small size, and are 

 frequently arranged in pairs or in groups of four. Sometimes 

 these groups of four or ' tetrads ' are disposed in a tetrahedral 

 manner, but at other times they are situated in one plane. Each 

 cell contains a somewhat bell-shaped chloroplast which may or 

 may not be furnished with a pyrenoid. The nucleus is situated 

 in the hollow of the chloroplast. In some genera (e.g. Tetraspora 

 and Apiocystis) each cell is furnished with a pair of ' pseudocilia,' 

 which consist of long motionless protoplasmic threads penetrating 

 through the enveloping mucus to the exterior. These were first 

 discovered by Thuret. 



Multiplication takes place by cell-division in two or three 

 directions, followed sooner or later by a diffluence of a large part 

 of the enveloping mucus and the consequent dismemberment of 

 the colony into smaller portions, each of which increases as before 

 either by simple cell-fission or by the formation of two or four 

 daughter-cells within each mother-cell. 



Asexual reproduction takes place by biciliated zoogonidia. 

 The latter are formed either by the transformation of a vegetative 

 cell into a zoogonidangium in which several zoogonidia arise, or 

 by the assumption by the ordinary vegetative cell of the motile 

 Chlamydomonadine condition. The motile state greatly resembles, 

 the Chlamydomonad-type, and these plants have no doubt arisen 

 by the intercalation of a simple though well-marked vegetative 

 condition between two successive motile phases. 



Sexual reproduction has been observed in some of the Palmel- 

 laceae. It consists of a fusion of isogamous planogametes, either 

 similar in all respects to the zoogonidia and produced singly in a 



