MULTIPLICATION IN PLANTS 



471 



able to start a new growth and in being able to await favorable 

 conditions for growth. It is unlike a spore in being produced 

 by the union, or conjugation, of two preexisting cells, not by 

 a division. Conjugation takes place in the black mold and 

 many other fungi, as well as among many of the simpler algae. 



^mm^^^ 



Fig. 193. 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, d\ the 

 mass of protoplasm 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 com- 

 pletely 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 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 



The cells which take part in conjugation are called gametes, 

 from a Greek word meaning "to marry," that is, to join, or unite 

 with. The cell which is formed by the union of the two gametes 

 is called a zygospore, that is, a spore resulting from a joining, 

 or yoking together. We may call it zygote for short. 



