xxi HISTOLOGY 209 



There is frequently found springing from the axil of one of 

 the leaves in a whorl a branch or shoot (br) which repeats 

 the structure of the main stem, i.e. consists of an axis from 

 which spring whorls of leaves, the whole ending in a ter- 

 minal bud. The axis or stem of a shoot is called a second- 

 ary axis, the main stem of the plant being the primary axis. 

 It is important to notice that both primary and secondary 

 axes always end in terminal buds, and thus differ from the 

 leaves which have pointed extremities. 



The rhizoids or root-filaments (rJi) arise, like the leaves 

 and branches, exclusively from nodes. 



In the autumn the more distal leaves present a peculiar 

 appearance, owing to the development on them of the gonads 

 or sexual reproductive organs (Fig. 45, B and G) : of these 

 the spennaries (antheridia) look very like minute oranges, 

 being globular structures (spy) of a bright orange colour : 

 the ovaries (oogonia) are flask-shaped bodies (ovy) of a 

 yellowish brown colour when immature, but turning black 

 after the fertilization of the ova. 



Examined under the microscope each internode is found 

 to consist of a single gigantic cell (F, int. nd' 2 ) often as much 

 as 3 or 4 cm. long in the older parts of the plant. A node 

 on the other hand is composed of a transverse plate of small 

 cells (nd 1 ) separating the two adjacent internodes from one 

 another. The leaves consist each of an elongated proximal 

 cell like an internode (D, /; F, 7 1 ), 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 arrangement. 



The details of structure of a single cell are readily made 



