310 



MANUAL OF BOTANY 



Fig. 667. 



modes of cell-production belong to this categorj'. The first is 

 known as rejuvenescence, and is met with only in connection 

 with reproductive cells. It consists in the protoplasm of a cell 

 withdrawing itself from the cell-wall, and after a short period of 

 quiescence secreting for itself a fresh membrane. It may remain 

 within the original cell- wall, as in the formation of spores inside 

 the special mother-cells, which have themselves been formed as 

 above described by cell- division. On the other hand, it may be 

 set free. In this case the cell- wall is not secreted by it until 

 after its escape. It is, moreover, at first usually furnished with 



cilia, sometimes at- 

 tached to one end as in 

 the zoospores of CEdo- 

 gonium (fig. 666, c), or 

 over the whole surface 

 as in the zooccenocytes 

 of VaucJieria. 



The other mode of 

 cell-production without 

 cell-division isthatof the 

 fusion of two cells to form 

 a new one. Like the last- 

 described method, it oc- 

 curs only in connection 

 with the processes of re- 

 production. It includes 

 all the cases of the 

 fusion of gametes al- 

 ready described. The 

 cells taking part in the 

 fusion are at the time 

 naked cells, consisting of 

 protoplasm and nucleus. 

 They may be similar or 

 dissimilar in size, may 

 be set free from the cells 

 in which they were 

 formed, as in the case 

 of the ciliated gametes 

 of Ulothrix {fig. 609) or 

 the spermatozoids of 

 the higher Cryptogams, or may leave their parent cell only to 

 pass into that of another gamete, as in Spirogyra (fig. 486) 



Fig. 667. Uogonia and antheridiaof Aclilya liffiii- 

 cola, showing cell-division. The letters A to E 

 indicate the course of development. The 

 protoplasm of a cell or branch of a cell collects 

 into a globular form, A, B, and by the forma- 

 tion of ji septum, D q, becomes an independent 

 cell (the oogonium). The protoplasm then 

 breaks up into two or more parts, n, e, e 

 (oospheres), which quickly become spherical, 

 and after possible fertilisation by the antheridia 

 rt, a, penetrating into the oogonium by their 

 sacs, h, 6, as seen in D, secrete a cell-wall E, 

 and become oospores. After Sachs. 



