28 



I'LANT MORPHOLOGY 



strains, designated as plus and minus. A zygote may be formed only 

 by the union of a plus gamete with a minus gamete. Of the four zoo- 

 spores arising from the zygote, two belong to the plus strain and two 

 to the minus. These strains may soon undergo another sexual fusion 

 or may be perpetuated asexually for an indefinite period. In Chlamy- 

 domonas hraunii and a few other species the gametes of the plus strain 

 are slightly larger than those of the minus strain, and so here a visible as 

 well as a physiological sexual differentiation exists. Such species show 

 that Chlamijdomonas displays a slight tendency toward heterogamy. 



A ' B 



Pig. 17. Two species of Gonium, X900. A, side and top views of four-celled colony of 

 Gonium sociale; B, top view of sixteen-celled colony of Gonium pectorale. 



In Sphaerella, a close relative of Chlamijdomonas and common in rain- 

 w^ater pools, the inner portion of the cell wall is gelatinous and thick and 

 is traversed by many delicate cytoplasmic strands. Generally it con- 

 tains a bright red pigment, haematochrome , that masks the chlorophyll. 

 This is present in the cell sap. 



Gonium. This is a colonial form, each colony consisting of a flat plate 

 of cells numbering either four or sixteen, according to the species (Fig. 17). 

 The cells are regularly arranged and held together by a mucilaginous 

 matrix derived from their cell walls. Each cell is biciliate and otherwise 

 similar to an adult Chlamijdomonas. By division of its protoplast, any 

 cell may form a new colony that escapes from the parent cell. Sexual 

 reproduction occurs by the fusion of similar gametes (isogametes), the 

 two coming from separate colonies. The number of gametes formed in 

 a cell corresponds to the number of cells in the colony. They escape 

 separately. The zygote becomes thick-walled and dormant. Later it 

 produces four biciliate zoospores. In the four-celled species these usu- 

 ally remain together as a colony; in the sixteen-celled species they separate 

 and each forms a new colony. 



Pandorina. This form is similar to Gonium except that the colony is 

 spherical or nearly so and consists usually of 16 biciliate cells crowded 



