lO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



branches. N. violaceum, on the other hand, varies in the 

 direction of the finely dissected type of frond. JV. fryeamim 

 is a membranous form which often becomes narrowly lacin- 

 iate, with the segments much prolonged. JV. andersoni- 

 anuni, which has been instanced as an example of a finely 

 dissected frond, becomes, when growing in quiet water, very 

 broadly linear, with the amount of branching largely reduced, 

 and the expanded branches -very regularly arranged. 



Form and Branching as Specific Characters. — Some allu- 

 sion has been made in the foregoing pages to the value of 

 such characters as form and branching of the erect frond 

 for distinctive purposes in describing a species. The ques- 

 tion was raised then as to the advisability of making too 

 narrow limits for a species upon such distinctions as form 

 and branching, without regard to the effect of environ- 

 mental factors. The remarkable variations in these two 

 respects existing within the limits of a single species empha- 

 size this fact, and further call attention to the influence of 

 environmental relations upon form. 



Stalk. — The stalk, which characterizes many species, 

 varies in length, width and thickness. The plant, by vari- 

 ations in the length of the stalk, may be sessile, subsessile, 

 or long-stalked. In some forms the stalk is narrow and 

 somewhat thickened; in others it is furnished with a thin, 

 expanded margin. It may vary in thickness from a few 

 cell-layers to a thick, almost fleshy tissue. A midrib, when 

 present, usually runs through the median portion of the 

 stalk. Increase in thickness may take place through a growth 

 of the superficial cells, or through an increase in the number 

 of cells composing the central la)'^er. Frequently the thin 

 margin becomes worn away, and this is accompanied by an 

 increase in thickness of the median portion of the frond, so 

 that the stalk becomes cylindrical. 



Midrib. — The midrib presents considerable variation in 

 form and extent. In some species it may be distinguished 

 only as a slight thickening of the median portion of the 

 stalk or frond, while in others it is more highly differenti- 

 ated, appearing as a ridge of considerable prominence. It 



