250 Rhodora [Novbmbbb 



directed forward. There are four cilia, which are inserted close 

 together in a pit on the anterior end. One lip of the pit is higher 

 than the other. A large stigma occurs in the posterior third of the 

 cell and on its Hat face. The chloroplast is single and is usually 

 enp-shaped or sometimes cylindrical. At times it is seen to be two 

 or four lobed anteriorly, hut posterior lobes similar to those of /'. 

 tetrathele West were not. observed. The four mammillate anterior 

 projections of the cell of tetrathele are also usually lacking, though 

 in cells which have ceased to move the end view may sometimes he 

 quadrate. 



The pyrenoid in face view is cup-shaped, though it seems spherical 

 when viewed from the side. Starch grains are found around the 

 pyrenoid and at times elsewhere within the chloroplast. The single 

 nucleus occurs in the cavity of the chloroplast. 



The cell wall is a delicate membrane of cellulose, visible with 

 difficulty in the motile cell, though easily observed in fixed material 

 or in reproducing individuals. 



Reproduction (figs. 5-19) is exclusively asexual. A vegetative 

 cell loses its cilia and the contents divide into two by constriction. 

 Division may be either longitudinal, oblique, or transverse. The 

 first indication of division is an increase in the size of the cell followed 

 by the splitting of the chloroplast. Hie stigma then divides unequally, 

 the two parts soon separating. After this the protoplast divides and 

 last the pyrenoid. The peculiar shape of the pyrenoid seems to be 

 doe to its mode of division. Fission into equal halves takes place, 

 from which result two shallow cups. The edge of each cup grows 

 upward and inward as if to form a hollow sphere, but the process is 

 incomplete and a deep cup with a narrow opening results. 



In its structure and mode of division Platymonas tvbeordifonnis 

 shows a remarkably close resemblance to the zoospores of Prasino- 

 cladua subsaha (Davis) Wille. The resemblance is so striking as to 

 amount almost to identity. It would be difficult, to distinguish 

 between the two if they were mounted in the same drop. The size, 

 the thin membrane-like wall, the four cilia of the same length as the 

 cell, the conspicuous stigma, the plane of division varying from longi- 

 tudinal to oblique or transverse, the peculiar cup-shaped pyrenoid are 

 all identical. The zoospore of Prannocladus has not been described 

 as flattened, but this point is not. easily observed and may have been 

 overlooked as it was when Wille described Carteria wbcordiformis. 



