312 FRANCIS DARWIN. 



deiisations of cell-sap incapable of originating movement, 

 it follows as a necessary corollary that the movements are 

 impressed on these passive masses by protoplasm of some 

 kind external to them. We can therefore state the two 

 opposing theories to be discussed, thus : — 



I. My father's view, that the aggregated masses consist of 

 protoplasm, and that their movements are simply due to 

 their own contractility, excited by various external 

 agencies. 



II. Professor Cohn's view, which appears to be that the 

 aggregated masses consist of condensations of cell-sap, and as 

 a necessary corollary that the movements are impressed on 

 the masses by some kind of protoplasmic action external to 

 the masses. 



The received notion of the structure of a typical adult 

 vegetable cell is that within the cell- wall there is a sac of 

 protoplasm enclosing the cell-sap, and sending prolongations 

 in the form of " plates or threads," which traverse the cell- 

 sap cavity in various directions; the nucleus maybe described 

 as differentiated protoplasm. The typical cell may, therefore, 

 be said to contain two kinds of protoplasm. 



As far as the present discussion is concerned Strasburger's 

 classification of vegetable protoplasm^ confirms this state- 

 ment, although in reality he makes a third variety, his 

 "couche membraneuse," which includes the peripheral or 

 limiting portion of a protoplasmic mass. 



A cell of Tradescantia obviously presents the two main 

 varieties of protoplasm, namely, those which form the current 

 and the nucleus, the rest of the cell being occupied by purple 

 cell-sap. But; if my father's view is correct, the Drosera 

 cell not only differs in having lost its nucleus, but also in 

 possessing another variety — a coloured protoplasm diffused 

 throughout the cell, and distinct from the granular proto- 

 plasm which forms the current. 



Professor Strasburger remarks "that the separation of the 

 protoplasm into the granular plasma, the membranous layer, 

 and the nucleus signifies a division of labour, in such a way 

 that the nucleus governs the molecular phenomena con- 

 nected with the genesis of cells, while the membranous layer 

 is charged with the peripheral limitation of the whole, and 

 the granular layer (or plasma) with nutrition."^ 



Now, the cells of the tentacle of Drosera, besides the 

 vital functions common to all cells, have special ones to per- 

 form, namely, absorption of a special kind of food and the 



1 ' Sur la Formation, &c., des Cellules,' 1876, p. 263. 

 « Loc. cat., p. 263. 



