NITELLA FLEXIL1S. 105 



resemble N. translucens, but can usually be at once dis- 

 tinguished by the comparatively long secondary rays, each 

 consisting of a single cell. Dried specimens have never the 

 peculiar sheen which characterises N. trauslucens. 



The points of resemblance between N. opaca and N. flexilis 

 are so numerous and close that the distinction between the 

 two species can only be determined in the last resort by their 

 sexual difference. In consequence, in their barren state 

 precise determination is almost impossible. 



Even sexually the two species tend to approximation, as is 

 seen in var. Fryeri, which is markedly protandrous. Professor 

 Hy in his " Characees de France ; refers to a plant found in 

 Lake Pavin (Puy de Dome) which he feels compelled to assign 

 to N. flexilis rather than to N. opaca in spite of its apparent 

 dioecious character. 



The characteristics which normally distinguish the two 

 species may be set forth as follows : N. opaca is smaller and 

 stouter, with denser heads; at the same time, stout forms of 

 N. flexilis are not uncommon, var. crassa being very thick - 

 stemmed, and var. nidiflca forming dense heads ; while, con- 

 versely, var. attenuata of N. opaca has an extremely slender 

 diffuse habit. The terminal rays of N. opaca are usually 

 2-3 and end in an abrupt mucronate point; while in N. 

 flexilis the terminal rays are sometimes single and tend to be 

 acuminate. In N. opaca the an theridium is larger (650-775 /u) 

 than in N. flexilis (500-625 /t) ; while conversely the oospore 

 of N. opaca is on an average considerably smaller (375- 

 425 JLI) than in that of N. flexilis (500-575 ju). N. flexilis 

 produces its fruit later than N. opaca. 



In spite of the similarity of the two species their distribu- 

 tion is very different. While N. opaca is found in almost every 

 county and vice-county, N. flexilis has been observed in forty- 

 eight only. 



The character of the membranes of N. opaca and N. flexilis 

 is difficult to determine with precision. In some fruits the 

 membrane is apparently quite smooth and clear, in others 

 the upper layer of the membrane is smooth while the under 

 layer or layers present a rorid or mottled appearance as 

 though constituted of granules which have become swollen 

 and distended and have assumed papillary forms. In many 

 cases this sub-surface development does not rise to the surface, 

 in other examples it breaks through the top layer of the mem- 

 brane and then exhibits wart-like eruptions or protuberances. 

 These appear to be smaller, more evident and more numerous 

 in N. flexilis than in N. opaca. 



