ARCHER — ON CONJUGATION IN SPIROTiENIA. 149 



I have not yet been able to detect a nucleus in any other species of 

 Spirotsenia ; perhaps, as in S. condensata, it requires favourable circum- 

 stances to reveal it. I am, however, the more desirous to draw atten- 

 tion to it as it exists in the species under consideration, inasmuch as it 

 forms a seemingly noteworthy exception to other Desmidiacese in this 

 regard. In all other species in which the nucleus can be seen it is 

 orbicular and central; nor does de Bary, in his work on the Conjugatse, 

 draw attention to the peculiarity in this species which I have pointed- 

 out — nay, his figure* leads to the idea that he regarded the nucleus as 

 central ; but this may, indeed, arise from his having seen and drawn it 

 either from above or below, and not from the side, which, as I have 

 shown, would be deceptive. 



Another reason which causes me to think it advisable that atten- 

 tion should be drawn to the form and position of the nucleus in this 

 species is the possibility that observers might imagine, upon casually 

 viewing an example, that it perhaps represented nothing but a detached 

 joint of a Spirogyra. Such a mistake, indeed, I could hardly imagine 

 possible when sufficiently closely examined ; but even if it he possible, 

 I think, due regard being had to the circumstance that the nucleus in 

 Spirotcenia condensata is semiorbicular and parietal, whilst in Spirogyra 

 it is equally compressed and central, ought at once to preclude the 

 chance of any confusion. 



To pass on to the conjugated state. 



When I first examined the gathering, in which this species occur- 

 red more than usually copiously, my attention was attracted by the 

 number of cells lying side by side over the field of view in parallel 

 pairs. Under such circumstances it is always well not to lose sight of 

 the specimens of whatever species maybe so encountered, as it betokens 

 impending conjugation ; accordingly I placed these aside for further 

 observation. Nor was there any disappointment in this case. 



The following is the process : — 



Shortly the cell-contents of each opposite parent cell so lying side 

 by side become separated into two portions, which by degrees become 

 more and more contracted into a shorter and shorter elliptic mass. 

 As the contraction of each half of the contents of each cell advances, 

 the spiral arrangement becomes more and more obliterated, until finally 

 there is little or no trace left of the original . spiral band (PL VIII. , 

 fig. 5.). 



It is to be regretted that the observation is here insofar incomplete 

 that I can give no record of what becomes of the nucleus during this 

 process. Even though it disappeared along with the separation of the 

 cell-contents into two portions, and were there actually a new nucleus 

 formed in each half, they could not be indeed now seen, as the green 

 contents become so much more densely packed than when, as a spiral 

 band, they occupied the whole cavity of the parent cell. 



* Op. cit., t. v., Fig. 12. 

 VOL. V. X 



