228 GENERAL BOTANY 



this process the cytoplasm of the two gametes fuse first, and the 

 two gamete nuclei, called prcwudei, fuse later to form the one 

 double conjugate nucleus.. As soon as the gametes unite, the 

 double cell formed by the union of the gametes rounds up 

 and secretes a cell wall, which completes the formation of the 

 zygote (e). Since this zygote in Spirogyra is designed to carry 

 the plant over inclement periods of drought or cold, it develops 

 a very heavy protective cell wall, which is divided into three 

 layers. The outer layer is thick and protects the protoplast from 

 mechanical injury and sudden changes of temperature ; the middle 

 coat is impervious to water and prevents its loss from the pro- 

 toplast during drought if the pond or stream in which it lives 

 happens to dry up in summer; the inner coat is thin and elastic. 

 Pyrenoids are also abundant in the mature zygote and, with the 

 aid of the microscope, are easily distinguished from the nucleus 

 by the central granule and surrounding starch sheath. 



After its rest period germination of the zygote (c?) takes place, 

 owing to environmental stimuli. In the germination process the 

 zygote absorbs water, and the swelling of its contents results in 

 the rupture of the two outer protective layers of the cell wall, 

 which allows the inner elastic layer to protrude and elongate 

 into a filament. This elongation of the zygote cell is accom- 

 panied by cell and nuclear divisions which result in an offspring 

 in the form of a minute Spirogyra plant in which the chloroplasts 

 are early reorganized and the nucleus takes up its central posi- 

 tion in each cell. The embryo plant thus begins to develop by 

 growth and cell division into an adult Spirogyra plant, which 

 completes the process of reproduction, and the new plant enters 

 upon a period of independent vegetative life. 



LIFE HISTORIES OF ALG^E 



Every kind of organism, during its life, passes through certain 

 stages and processes which, taken together, are termed its life 

 history. The life history may therefore be conveniently considered 

 and graphically illustrated as a cycle of events in the life of an 

 individual organism which follow each other in orderly sequence. 



