196 SPIROGYRA less. 



ing the nucleus («?/), two spiral chromatophores {chr), and pyrenoids 



b', b'', middle portion of a cell, showing two stages in binary 

 fission. 



c, four stages in dioecious conjugation : in c' the gonads {gon^, gon"^) 

 are connected by short processes of their adjacent sides : in c'-^ the active 

 or male gamete {gam^) has separated from the wall of the gonad igon^) 

 preparatory to passing across the connecting bridge to the stationary or 

 female gamete {gam^) which has not yet separated from its containing 

 gonad (gon-) : in c* the female gamete [ganfi) has undergone separation, 

 and the male gamete (gam^) is in the act of conjugating with it : in c* the 

 two have united to form a zygote (syg) lying in the female gonad. 



D, two stages in monoecious conjugation : in D^ the adjacent cells 

 (gonads) have sent out conjugating processes (a) : in D- conjugation is 

 complete, the male gamete having passed through the aperture between 

 the conjugating processes and united with the female gamete to form 

 the zygote (syg). 



E, parthenogenetic formation of zygotes. 



F, fully developed zygote (zygospore). 



G, early stage in the germination of the zygote, 



(b after Sachs : c after Strasburger : F and G from Sachs after 

 Pringsheim. ) 



larger cells of plants the development of vacuoles is so ex- 

 tensive that the protoplasm is reduced to a thin layer in 

 contact with the cell-wall (see pp. 169 and 188). This state 

 of things is carried to excess in Spirogyra : the central vacuole 

 is so large that the protoplasm (a, p/sm) has the character 

 of a mere delicate colourless membrane within the cell-wall : 

 to make it out clearly the specimen should be treated with 

 a fluid of greater density than water, such as a 10 per cent, 

 solution of sodium chloride, which, by absorbing the water 

 in the vacuole, causes the protoplasm to shrink away from 

 the cell-wall and so brings it clearly into view. It is to this 

 layer of protoplasm that the name primordial utricle is 

 applied by botanists, but the student should remember that 

 a primordial utricle is not a special constituent of those 

 cells in which it occurs, but is merely the protoplasm of a 

 vegetable cell in which the vacuole is inordinately large. 

 The protoplasm of the cell of Spirogyra is not, however. 



