174 



CRYProGAMS 



419. GroirtJi and reproduftion should now be clearly 

 distiiiyuis'iLed. Growth is the increase in size of an 

 already existing individual ; rcproduclion is the forma- 

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

 case of Plenrococcus cell division results in the produc- 

 tion of two new indiviiluals, 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 Jilainejit to be the individual, then division of the 

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

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

 essentially independent <irganisms, their division must 

 then be regarded as reproduction. The two processes here 

 run together, since it is not cas}- to say how much of the 

 plant may be termed the individual. 



420. lieprodnction. — Under certain 

 conditions, however, the cells of iS[>i- 

 rogyra take part in a distinctly }'epro- 

 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. (.)penings are then 

 made in the conjoined outgrowths, by 

 \Nhich the contents of all the cells on 

 one side pass over into those on the 

 other. The contents of each pair of 

 cells unite to make up a single body, 

 287. Conjugation of or 2;/(7os/:>ore (ss), which becomes invested 



Spirogyra: ,cs, , " '' , . , ,, . 



zygospore: i\ ''.V '* tlucK wali preparatory to a restnig 

 fiisKJii lu pro- period. In this form the plant endures 



'•"rcss. 



periods of drought, when tlie pools 



where it grows dry up; and thus it also passes the wint(^r. 



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



reproduction. In plants soon to be (h^scril)ed the fusing 



cells differ largely in size and other characteristics. 



