198 SPIROGYRA less. 



Any of the cells of a Spirogyra filament may divide in this 

 way, so that the filament grows by the intercalation of new 

 cells between the old ones. This is an example of interstitial 

 growth. Note its difference from the apical growth which 

 was found to take place in Penicillium (p. 190), a difference 

 which explains the fact mentioned above (p. 194) that there is 

 no distinction between the two ends of a filament of Spirogyra, 

 while in Penicillium the proximal and distal ends can always 

 be distinguished in a complete hypha. 



The sexual reproduction of Spirogyra is interesting, as 

 being intermediate between the very different processes which 

 were found to obtain in Mucor (p. 165) and in Vauclieria 

 (p. 172). 



In summer or autumn adjoining filaments become arranged 

 parallel to one another and the opposite cells of each send 

 out short rounded processes which meet (Fig. 42, c^), and 

 finally become united by the absorption of the adjacent walls, 

 thus forming a free communication between the twoconnected 

 cells or gonads (gon^, gon^). As several pairs of cells on the 

 same two filaments unite simultaneously, a ladder-like ap- 

 pearance is produced. 



The protoplasmic cell-bodies (c^, gam^, gam-) of the two 

 gonads become rounded oft' and form gametes or conjugating 

 bodies (see p. 166, note i) : it is observable that this process 

 of separation from the wall of the gonad always takes place 

 earlier in one gamete (c^, gatn'^) than in the other (c^, c^ 

 garn^). Then the gamete which is ready first (gam^) passes 

 through the connecting canal (c^) and conjugates with the 

 other (gam"^), forming a zygote (c*, zjg) which soon surrounds 

 itself with a thick cell-wall. It has been ascertained that the 

 nuclei of the gametes unite to form the single nucleus of the 

 zygote. 



