20 A TEXT-BOOK OF BOTANY. 



gyra filament or plant. This method of reproduction is known 

 as CONJUGATION^ and the zygospore is called a resting spore. It 

 should be explained that certain cells, as well as spores, may lie 

 dormant for a period, as during the winter season or at other 

 times, when the conditions are unfavorable ta grow^th, and then 

 renew their activities, these being known as " resting cells." 



Chlorophyce.?'. — The Chlorophycese, or Green Algae, are dis- 

 tinguished by usually having a green color, due to chlorophyll, and 

 by having no other pigment. The cells contain one or more nuclei. 

 They are either unicellular or made up of many cells forming fila- 

 ments or flat sheets. They occur either singly as simple cells or 

 in groups representing a single individual or a colony. They are 

 found mostly in fresh or salt water, usually being microscopic 

 in size so as not to be noticed, but often attracting attention when 

 they occur in sufficient quantity to form a scum on the surface. 

 The reproduction is mostly by ciliated cells called zoospores or 

 swarm spores. Reproduction also takes place by the union of 

 the zoospores and through the fertilization of Qgg cells. The 

 sexual spore resulting from this union of like cells (forming 

 a zygospore) or of unlike cells (forming an oospore) seldom 

 develops immediately, but usually undergoes a resting period be- 

 fore growth is continued 



Protococcus. — One of the commonest of the Green Alga^ 

 as well as one of the simplest is Protococcus (Pleurococcus) vul- 

 garis (Fig. lo). It occurs as a green coating, in both winter and 

 summer, on the moist bark of trees, moist ground, and stone 

 walls, and is a component of some lichens. The plant is i- 

 celled, more or less spherical, and at one stage contains a number 

 of chlorophyll grains which finally unite to form a single plate 

 which lies against the wall and is known as a chromatophore. 

 Besides it contains a considerable amount of oil. An allied 

 species (Protococcus viridis) contains the sugar erythrite. The 

 plant usually reproduces by simple division, — that is, one cell 

 or plant divides to form two. The divisions may continue by the 

 production of another cross wall, so that four cells result. Under 

 favorable conditions, division may take place by the formation 

 of still another wall at right angles to the other two. In this 

 way two, four and finally eight individuals arise which adhere 



