216 NITELLA 



by budding. The entire spermary appears to be a modified 

 leaflet. 



The ovary also arises as a single cell, but soon divides and 

 becomes differentiated into an axial row of three cells (Fig. 

 49, B 2 , ov, nd, stk) surrounded by five others (sp. c) which arise 

 as buds from the middle cell of the axial row (nd) and are 

 at first knob-like and upright (B 1 ). The uppermost or distal 

 cell of the axial row becomes the ovum (B S , B 4 , ov\ the 

 others the stalk (stk) and intermediate cells (nd, x) : the five 

 surrounding cells elongate, and as they do so acquire a spiral 

 twist which becomes closer and closer as growth proceeds 

 (compare B 1 , B 4 , and Fig. 46, G, ovy). At the same time the 

 distal end of each develops two septa (B 3 ) and, projecting 

 beyond the level of the ovum, forms with its fellows the 

 chimney or crown (cr) of the ovary. There is every reason 

 to believe that the entire ovary is a highly-modified shoot : 

 the stalk representing an internode, the cell, nd a node, the 

 spiral cells leaves, and the ovum an apical cell. 



Thus while the ciliate Infusoria and Caulerpa furnish ex- 

 amples of cell-differentiation without cell-multiplication, and 

 Spirogyra of cell-multiplication without cell-differentiation, 

 Nitella is a simple example of an organism in which com- 

 plexity is obtained by the two processes going on hand in 

 hand. It is a solid aggregate, the constituent cells of which 

 are so arranged as to produce a well-defined external form, 

 while some of them undergo a more or less striking differen- 

 tiation according to the position they have to occupy, and 

 the function they have to perform. 



Impregnation takes place in the same manner as in 

 Vaucheria (p. 170). A sperm makes its way down the 

 canal in the chimney-like crown of cells terminating the 



