THE ALGM 229 



It is evident that the simplest life history in plants would be 

 presented by such a simple organism as Protococcus, which repro- 

 duces by the division of a mother cell into two daughter cells, 

 each of which, by growth and separation, forms a plant like the 

 mother. The very rapid reproduction and spreading of Proto- 

 coccus, bacteria, and yeasts is effected by this mode of vegetative 

 reproduction. 



The life history of a plant with sexual reproduction alone or 

 with sexual reproduction combined with vegetative reproduction, 

 as in Spirogyra, also presents a comparatively simple cycle of 

 stages. In Spirogyra the nutritive and growth phases of the life 

 cycle are amply provided for by the highly organized cells of the 

 filaments, which may also be multiplied by vegetative reproduc- 

 tion. The nutritive phases usually continue throughout the sum- 

 mer months, when food is plentiful in the waters in which these 

 plants are habitually found. The sexual phase is inaugurated by 

 the formation of gametes, and terminates with the maturing of 

 the zygote resulting from the fertilizing act. After its normal 

 period of rest the zygote gives rise to an offspring in the form of 

 a new nutritive plant, which brings the life cycle back to its start- 

 ing point. In Chlamydomonas there is added to the vegetative 

 and sexual phases of reproduction an asexual phase, represented 

 by zoospores. In such plants the asexual zoospore stage usually 

 has the function of rapid reproduction of the species, while the 

 zygote, which results from sexual union, functions as a resting 

 cell during inclement periods or seasons, so that there is a divi- 

 sion of labor between the asexual and sexual phases in the life 

 history. The relation of the different phases in the life history of 

 a plant may be indicated as in the following outline of the life 

 history of Spirogyra. The student should substitute simple 

 diagrams for the written designations of the various stages. 



male male 



y/gametangia gametes \^ 

 Gameto-/ \ fertili- zygote gameto- 



phytes \. Xzation germination phyte 



N^ female female / 



gametangia gametes 



