238 GENERAL MORPHOLOGY OF PLANTS 



the same thread are also receiving gametes, except in rare cases. 

 All the cells of a single thread are likewise supplying gametes. 

 Usually there is no difference in the size of the supplying and 

 receiving cells, and it is impossible to say what the nature of the 

 gamete is until movement of the protoplasm from the supplying 

 gamete is taking place. While the sex of the threads appears 

 in these cases to be distinct, there is no differentiation in the size 

 and form of the egg and sperm, the receiving gamete correspond- 

 ing to the egg or female cell, while the supplying gamete corre- 

 sponds to the sperm or male cell. There are some species, how- 

 ever, in which adjacent 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. 182). Evidently here 

 the two sex elements are present in a single thread. These 

 species may also conjugate in a ladder-like manner. 



369. Germination of the zygospore. After a period of 



rest and when the conditions become favorable the zygospore 



Sporophyte germinates and a new thread is de- 



zygoteQ veloped from the fertilized egg. 



370. 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 



Diagram No. I. Illustrating 



the life cycle in the development reached again. The life history of 



of Spirogyra. Course of devel- 

 opment follows the direction spirogyra maybe epitomized as follows. 



indicated by arrows. Zygote 



equals fertilized egg, which in The zygospore (the fertilized egg) ger- 



this case is the zygospore. * to 



minates and produces the plant in its 



filamentous form, the vegetative phase. Growth and increase 

 take place by division and elongation of any or all of the cells. 

 Propagation or multiplication takes place by the breaking up 

 of the threads into separate threads. Sexual reproduction 

 takes place by conjugation, either by conjugating tubes be- 

 tween cells of two distinct threads, or by a tube forming a 



