174 



CRYPTOGAMS 



419. Growth and reproduction should now be clearly 

 distinguished. Growth is the increase in size of an 

 already existing individual ; reproduction is the forma- 

 tion of a new individual, or new individuals. In the 

 case of Pleurococcus cell division results in the produc- 

 tion of two new individuals, which separate sooner or 

 later. In the growing root tip of a Flowering Plant, 

 on the other hand, cell division is merely a step in the 

 formation of more root, and is therefore only a growth 

 process. In the case of Spirogyra, if we consider the 

 whole filament to be the individual, then division of the 

 several cells is to be regarded as growth. But if the cells 

 of the filaments are considered to be the individuals, i.e. 

 essentially independent organisms, their division must 

 then be regarded as reproduction. The two processes here 

 run together, since it is not easy to say how much of the 

 plant may be termed the individual. 



420. Reproduction. Under certain 

 conditions, however, the cells of Spi- 

 rogyra take part in a distinctly repro- 

 ductive process. The cells of a filament 

 send out lateral processes which meet 

 similar processes from cells of another 

 filament (Fig. 287). Cells thus become 

 united in pairs. Openings are then 

 made in the conjoined outgrowths, by 

 which the contents of all the cells on 

 one side pass over into those on the 

 other. The contents of each pair of 

 =-j I cells unite to make up a single body, 



287. Conjugation of O r zygospore (zs), which becomes invested 



Spirogyra : zs. -,,-, i , 



zygospore; f, uj a thick wall preparatory to a resting 



fusion in pro- period. In this form the plant endures 



periods of drought, when the pools 



where it grows dry up; and thus it also passes the winter. 



421. Here, as in Ulothrix, two similar cells unite in 



reproduction. In plants soon to be described the fusing 



cells differ largely in size and other characteristics. 



