Microthamniacece 91 



acuminate. The branches all arise immediately below a trans- 

 verse cell-wall, and at first appear as lateral outgrowths from the 

 upper end of a cell. There is also a marked tendency for the 

 branching to be unilateral. The parietal chloroplast is long, entire, 

 and occupies about two-thirds of the inner wall of the cell ; it con- 

 tains no pyrenoids. 



There are two species, M. Kiitzingianum Nag. and M. strictissimum Rabenh., 

 1863 [ = M. vexator Cooke, 1882]. The former species (fig. 32 A D) is much 

 more abundant than the latter, and is most abundant in the early spring. It 

 occurs in small pools and ditches, and likes peat. It is a small, much branched 

 plant, with short branches of one to six cells, the diameter of the branches 

 being 3 5 p. M. strictissimum (fig. 32 E) is a larger plant with an erect 

 thallus up to 4 mm. in height. The branches are much longer and more 

 rigid, giving the plant a very different appearance from M. Kiitzingianum. 

 The diameter of the cells is 4 p,. Fig. 32 E is drawn from one of the original 

 specimens sent by W. B. Turner to M. C. Cooke when the latter described 

 the plant as " M. vexator" 



Genus Gongrosira Kiitz., 1843 [inclus. Pilinia Kiitz. (in part)]. 

 The thallus is attached to a substratum by a mass of parenchyma- 

 tous cells formed by a confluence of creeping branches. From this 

 mass of cells, which may be one or many layers of cells in thick- 

 ness, arise numerous, erect, branched filaments, varying in height 

 from 0'04 to 2 mm. This dense, cushion-like mass of erect fila- 

 ments is frequently incrusted with carbonate of lime, and sometimes 

 forms quite a hard stratum. The cell-walls are often thick and 

 distinctly lamellose. The chloroplast is a parietal plate with one 

 or many pyrenoids, but it is usually difficult of observation. The 

 cells generally present the appearance of being filled with a dense 

 chlorophyllaceous mass, which has been proved to contain starch 

 other than that present in the pyrenoids. The zoogonidia arise in 

 terminal zoogonidangia which are generally flask-shaped. The 

 akinetes are ordinary cells, generally of the recumbent portion of 

 the thallus, which become detached and ultimately form new 

 plants. The plants usually occur at the margins of ponds, lakes, 

 or rivers, forming a tough green stratum on submerged stones or 

 on the shells of aquatic Gastropods. 



Species of this genus are rare in the British Islands, or perhaps they may 

 have been overlooked. G. viridis Kiitz. is a small species (thickness of prim. fil. 

 8 12 p., of branches 4 8 p ; fig. 33 A C) usually encrusted with lime. 

 G. stagnalis (West) Schmidle is a larger species (thickness of prim. fil. 



