74 A TEXTBOOK OF THEORETICAL BOTANY 



most advanced members there is an elaborate alternation of sexual and 

 asexual generations resembling more closely the Rhodophyceae than the 

 Chlorophyceae. Asexual reproduction is either by means of zoospores or 

 non-motile aplanospores. 



The two types selected for special study may be regarded as representmg 

 the two extremes of the group. Coleochaete is the most elaborate, in fact its 

 life history is in some ways more complex than that of any other Green Alga. 

 Pleurococcus, on the other hand, is a very specialized but reduced form which 

 has not only largely lost its filamentous character but also its reproductive 

 mechanism, and relies on vegetative multiplication of cells by simple division. 

 It would be difficult to select any genus which might be said to be typical 



of the order as a whole, and it 

 must be realized that neither of 

 the types selected for detailed 

 study fulfills that claim. 



Pleurococcus naegelii 



Pleurococcus is one of the 

 commonest of the Algae found 

 in terrestrial situations, occur- 

 ring as a thin incrustation on 

 the windward side of trees, stones, 

 walls and palings. Probably no 

 Alga has been subjected to more 

 discussion as to its correct name. 

 Frequently it is referred to as 

 Protococcm vin'ciis and also as 

 Chlorococcum viilgare, while ac- 

 cording to some workers its 

 correct name is Pleurococcus 

 vulgaris, though most authorities 

 seem now agreed that it should 



r:,^ t: r,, /a f he named Pkurococcus naepelH. 



riG. 50. — rleurococcus naegelii. A group 01 * 



adherent vegetative cells, some showing cell The Structure is extremely 



^''•'*""- simple (Fig. 56). The mature 



cells are sometimes isolated and more or less spherical, but they are more 

 frequently found in groups of two, three, four or more, owing to their slow 

 separation after division. In moist conditions there is a tendency for the cells 

 to remain attached to one another and to elongate, with the result that short 

 branched filaments are formed and such filaments readily arise when the 

 Alga is grown in culture flasks. 



Each cell (Fig. 57) is surrounded by a firm cell w^all which is usually 

 unthickened. The protoplast does not possess any obvious vacuoles and 

 contains a hollow, spherical chloroplast with, on one side, an opening with 

 a lobed margin. So large is this chloroplast that it seems to fill the cell, 



