Pftlmdhtcece 239 



Family 8. PALMELLACE^. 



The Palmellaceae is one of the most primitive families of green 

 Alga?, primarily distinguished from the other groups of the 

 Protococcoidea? 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- 

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

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



