298 



ELEMENTARY BIOLOGY 



Among some of the fresh-water algae the swimming cells 

 produced are of two sizes. Tn such cases the smaller cell is 

 usually more active in the water ; the larger cell has more food 

 material. In the common rockweed, or bladder wrack, of our 





Fig. 130. Conjugation in Spirogyra 



Cells lying close together put forth processes, or projections, toward each other, a. As 

 these processes finally come in contact, b, the two threads with their crosspieces have 

 the appearance of a ladder when looked at through the microscope, c. The cell walls at 

 the points of contact are dissolved, probably by the action of a ferment, and there are 

 thus formed continuous passages between the cells of one thread and the cells of the 

 opposite thread, d. In the meantime, however, changes have been taking place inside 

 the cells. The spiral ribbon of chlorophyl seems to break down, <f •. the mass of proto- 

 plasm in each cell draws away from the cell wall : and the protoplasm from one of the 

 cells of each pair moves into the connecting tube and passes completely into the opposite 

 cell, e. Here the two masses of protoplasm unite into one, and a thick cell wall is formed 

 around the new combined protoplasm, /. The cell with the thick wall, inside the old 

 dead cell wall, may apparently remain in a resting state indefinitely, or may begin the 

 next day to put out a thread of new Spirogyra, giving rise to millions of cells in the 



course of a few days 



seacoast, the gametes are produced in special organs found on 

 certain of the bladderlike expansions of the plant body. When 

 formed, the gametes are discharged into the water and have 

 nothing more to do with the parent plant (Fig. 131). 



348. Fertilization. When the two gametes are so unlike as 

 to be distinguishable, the process of conjugation is sometimes 



