2l8 GENERAL MORPHOLOGY OF PLANTS 



and sperm, the receiving gamete corresponding to the egg or 

 female cell, while the supplying gamete corresponds to the sperm 

 or male cell. There are some species, however, in which adja- 

 cent cells in the same thread conjugate by lateral tubes at the 

 adjacent ends which bend toward each other and fuse in the 

 form of a buckle joint (fig. 173). Evidently here the two sex ele- 

 ments are present in a single thread. These species may also 

 conjugate in a ladder-like manner. 



356. Germination of the zygospore. After a period of 

 rest and when the conditions become favorable the zygospore 

 germinates and a new thread is developed from the fertilized egg. 



357. Life history of spirogyra. The . life history of a 

 plant is an account of its development from the egg, or from some 

 starting point, through its different forms, including the means for 

 propagation and reproduction, until the egg, or the same starting 

 point, 'is reached again. The life history of spirogyra may be 

 epitomized as follows. The zygospore (the fertilized egg) ger- 

 minates and produces the plant in its filamentous form, the vege- 

 tative phase. Growth and increase take place by division and 

 elongation of any or all of the cells. Propagation or multiplica- 

 tion takes place by the breaking up of the threads into separate 

 threads. Sexual reproduction takes place by conjugation, either 

 by conjugating tubes betw r een cells of two distinct threads, or by a 

 tube forming a buckle joint connecting two adjacent cells of the 

 same thread. The zygospore is formed in one of the two con- 

 jugating cells by the fusion of the contents of the two cells into one, 

 the shrinkage of this mass from the wall forming a rounded body 

 with a thick wall within the female gamete case, or egg case. 

 The fusion of the two nuclei in the zygospore completes the proc- 

 ess of fertilization of the egg which passes through a resting stage. 

 The life history is sometimes spoken of as a life cycle. It may be 

 represented by the diagram or by the following formula: * 



Plant, veg. ^ . \ zygospore = fertilized egg > Plant, etc. 



\ female gamete '. 



* This formula, and those which follow other groups of plants, are not to be 

 memorized by the pupil. It is simply presented to serve as a graphic repre- 

 sentation of the life cycle. 



