Pleurococcacew 



203 



by the formation of one or many spores (aplanospores), by the 

 rejuvenescence of the contents of a mother-cell, by biciliated 

 zoogonidia, and by isogamous gametes. The polymorphism 

 exhibited by plants of this genus under various conditions of 

 humidity, temperature, etc., has caused much confusion with 

 regard to the identity of the different forms. 



PL vulgaris Menegh. is one of the commonest of Algse, occurring in great 

 profusion in all kinds of damp situations, on stones, walls, palings, tree-trunks, 

 etc., and it usually forms a thin green incrustation on the windward side of 

 the objects on which it grows. The cells are as described for the genus and 

 the chloroplast is a massive lobed plate containing a prominent pyrenoid. 

 Diam. cells 9 20 p ; fig. 81 A. In PI. rufescens (Kiitz.) Breb. the cell-contents 

 are of a brick-red colour due to the presence of hpernatochroinin, which usually 

 appears to be dissolved in oily material. This species has a preference for 

 calcareous rocks ; a variety of it (var. sanguineus West & G. S. West) has 

 been observed in the limestone districts of West Yorkshire, forming large 

 brilliant blood-red patches, covering those stones and rocks in the beds of 

 streams which could not be displaced by the rapidity of the current and 

 which are often left dry. Diam. cells 1120 p ; fig. 81 B. 



Genus Trochiscia Kiitz., 1845. [Acanthococcus Lagerh., 1883 ; 

 Glochiococcus De Toni, 1888.] This genus is very closely related 



K 



Fig. 82. A F, Trochiscia aspera (Eeinach) Hansg., from Tremethick, Cornwall ; 

 A and B, vegetative cells ; C and D, formation of zoogonidia ; E, empty cell 

 from which zoogonidia (zg) have escaped ; F, palmelloid state. G and H, 

 T. hirta (Beinsch) Hansg., from Cambridge. I and J, T. panci.ipinosa West, from 

 Ben Lawers, Perthshire. K, T. reticularis (Beiusch) Hansg., from Keston 

 Common, Kent. (All x 520.) 



to Pleurococcus, differing mainly in the external ornamentation of 

 the cell-wall and in the aquatic habit. The cells are perfectly 

 globose and usually occur in large aggregates in quiet water, or 

 rarely on damp ground. The cell-wall is either areolated or 

 thickly clouhed with denticulations, spines, or other prominent 



