STRUCTURE.] THWAITES's THEORY. 43 



gelatine or mucus of definite diameter, showing that in this 

 genus the amount of gelatine depends upon the number of 

 cells. 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 

 circumstances under which the plant grows. It occurs in 

 some situations in the form of a mucous stratum upon the 

 surface of stone ; in others the gelatinous sheath is of extreme 

 tenuity and transparency : whereas, if the plant is found in 

 rather deep rapid streams, the sheath is much developed, and 

 becomes of almost membranous texture ; thus showing that 

 this gelatinous structure is of subordinate character, and may 

 vary according to the circumstance in which the plant is 

 found. " If these statements are true of Algals, the same 

 explanation would apply to higher plants ; and Mr. Thwaites 

 regards it as not improbable, that the deposits of sclerogen as 

 well as the firm portion of the spiral fibre may be considered 

 as structures of a similar character. "The pellicle which 

 covers the epidermis is doubtless so. Now what is the cha- 

 racter of the mucus which we have seen to be developed in 

 definite quantity outside and around the cell-wall ? That it 

 is not a mere chemical solution of starch would appear 

 evident from its persistence when mounted for the microscope 

 in water and other fluids. Its toughness and elasticity, the 

 readiness with which it allows water to permeate it, and its 

 recovering its original form and consistence upon being 

 moistened after desiccation, seem to warrant the belief that 

 it possesses an organised form of the same mechanical 

 properties as sponge. 



' ' As, in viewing a series of Algse, a transition may be ob- 

 served from a mucous structure to one possessing the external 

 characters if not the functions of cell-membrane ; it may be 

 inferred that cell-membrane is of a similar mechanical struc- 

 ture, and we should perhaps not be far from a right definition 

 in applying to it the term felt, as indicating its real charac- 

 ters." These are the considerations that have led Mr. 

 Thwaites to the conviction " that cell-membrane is quite a 

 subordinate part of living structure ; that its functions are of 

 a purely physical character; that its principal office is to 



