20 Mr. G. H. K. Thwaites on the Cell-Membrane of Plants. 



Una, the original mucous sheath appears not to be absorbed, but 

 to be ruptured upon the production of new ones within it. Each 

 cell of some species of this family is surrounded by two or more 

 distinct mucous envelopes ; and in some species a cluster of cells 

 is also surrounded by a common mucous sheath, which is no 

 doubt also developed from the cells. In other species of the 

 Palmellece the cells are raised upon mucous prolongations caused 

 by the development of mucus on one side of the cell. The curved 

 moniliform filaments of the genus Nostoc would at first sight ap- 

 pear to grow in a mass of gelatine without any definite arrange- 

 ment ; but when, as is sometimes the case, the plant occurs with 

 a single straight filament, this is found to be surrounded by a 

 gelatine or mucus of definite diameter, showing that in this ge- 

 nus the amount of gelatine depends upon the number of cells. 

 That the gelatinous stipes of Cocconema, and therefore of the al- 

 lied genera, is developed from the frustules, is well- shown in a 

 curious state of Cocconema lanceolatum which I have recently 

 found. In this, each pair of frustules, instead of being raised 

 upon a long stalk, has become invested with a definite mucous or 

 gelatinous envelope of the same character as the short stipes to 

 which it is attached, and of which organ it would appear to be an 

 abnormal condition. In Schizonema the gelatinous sheath may 

 often be shown to bear a proportion to the number of frustules it 

 contains. In a freshwater species of Schizonema, occurring abun- 

 dantly in the neighbourhood of Bristol, the common mucous sheath 

 is liable to considerable modification according to the circum- 

 stances under which the plant grows. It occurs in some situations 

 in the form of a mucous stratum upon the surface of stones ; in 

 others the gelatinous sheath is of extreme tenuity and transpa- 

 rency : whereas, if the plant is found in rather deep rapid streams, 

 the sheath is much-developed and becomes of an almost mem- 

 branous texture ; thus showing that this gelatinous structure is 

 of subordinate character, and may vary according to the circum- 

 stances in which the plant is found. 



Microcoleus possesses a gelatinous sheath, but in the allied 

 genera Oscillatoria, Calothrioc, &c. this is represented by a truly 

 membranous sheath, closely resembling and no doubt identical 

 in function with cell-membrane. This fact, coupled with what 

 is observed during the formation of the spore of Zygnema, where 

 the endochrome seems at first to be held together by mucus, 

 would make it appear not unlikely that cell- membrane is really 

 a modification of a similar mucus or gelatine, and that the ulti- 

 mate structure of both is similar. 



In examining the fronds of some of the foliaceous Algse, it may 

 very readily be perceived that the cells composing it are separated 

 rom one another by the interposition of an apparently homo- 



