208 NITELLA 



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

 surrounded by a wall of cellulose (E, c.iv) of considerable 

 thickness. Within this is a layer of protoplasm (primordial 

 utricle, p. 194), enclosing a large central vacuole (vac), and 

 clearly divisible into two layers, an outer (plsm 1 ) in im- 

 mediate contact with the cell- wall, and an inner (plsm 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 -^^ mm. 

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

 opposite sides of the cylindrical cell are two narrow oblique 

 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 

 common in vegetable cells in which, owing to the confining 

 cell-wall, no freer movement is possible. 



The numerous nuclei (E, mi) are rod-like and often curved : 

 they can only be seen to advantage after staining (Fig. 47). 

 Lying as they do in the inner layer of protoplasm, they 

 are carried round in the rotating stream. 



In the general description of the plant it was mentioned 

 that the stem ended distally in. a terminal bud (Fig. 46, A, 

 term, bud), formed of a whorl of leaves with their apices 

 curved towards one another. If these leaves (F, 7 1 ) are dis- 



