xx HISTOLOGY 207 



cells (nd l ) separating the two adjacent internodes from one 

 another. The leaves consist each of an elongated proximal 

 cell like an internode (D, /; F, /*), then of a few small cells 

 having the character of a node, and finally of two or three 

 leaflets (D, G, /'), each consisting usually of three cells, the 

 distal one of which is small and pointed. 



Thus the Nitella plant is a solid aggregate in which 

 the cells have a very definite and characteristic arrange- 

 ment. 



The details of structure of a single cell are readily made 

 out by examining a leaflet under a high power. The cell is 

 surrounded by a wall of cellulose (E, c.w] of considerable 

 thickness. Within this is a layer of protoplasm (primordial 

 utricle, p. 196), enclosing a large central vacuole (vac), and 

 clearly divisible into two layers, an outer (pls/n l ) in im- 

 mediate contact with the cell- wall, and an inner (ptsw 2 ) 

 bounding the vacuole. 



In the outer layer of protoplasm are the chromatophores 

 or chlorophyll- corpuscles (chr) to which the green colour of 

 the plant is due. They are ovoidal bodies, about T J^ mm. 

 long, and arranged in obliquely longitudinal rows (D). On 

 opposite sides of the cylindrical cell are two narrow ob- 

 lique bands devoid of chromatophores and consequently 

 colourless (D). The chromatophores contain minute starch 

 grains. 



The inner layer of protoplasm contains no chlorophyll 

 corpuscles, but only irregular, colourless granules, many of 

 which are nuclei (E, nu : see below, p. 211). If the tem- 

 perature is not too low this layer is seen to be in active 

 rotating movement, streaming up one side of the cell and 

 down the other (E), the boundary between the upward and 

 downward currents being marked by the colourless bands 

 just mentioned, along which no movement takes place (D). 

 This rotation of protoplasm is a form of contractility very 



