THE CONJUGATION OF THE DIATOM RHABDONEMA ARCUATUM. 137 



5. The sporangial or zygospore-frustule consists of two valves, 

 without annuli, which have a length ahout thrice that of the valves 

 of the female. 



With regard to the inducing causes of conjugation it would 

 appear that in the present species self-division, which gradually 

 reduces the size of the bounding valves, has gone on so long that 

 a new generation becomes necessary to maintain the size. It can 

 scarcely be induced by any extraneous influence, for the examples 

 of conjugation occur amidst a profusion of vigorously-growing 

 filaments. I have failed to discover any visible cause of the selec- 

 tive adhesion of the conjugating individuals. If there were any 

 viscous coating on the female frustules the accidental appulse of a 

 male frustule might fix it sufficiently to destroy its slight cohesion 

 to its fellows by its one isthmus. I have generally seen the male 

 filaments in the close neighbourhood of a conjugated specimen, and 

 it may be that fortuitously touching the female the attachment is 

 thus induced. 



In conclusion, permit me to remind you that the Diatomacece 

 have been classed as the highest division of the Conjugates in which 

 no sexual difference has been perceptible. In Rhabdonema, with 

 its composite frustule, we have seen that the conjugating process 

 is also (so to speak) composite as well as distinctly sexual. We 

 seem to have here a step towards — hardly a connecting link with 

 — the sexual process in those Algce which produce antherozoids. 



DESCRIPTION OF THE PLATES. 



Plate VII. 

 Fig. 1. — Free valve of Rhabdonema arcuatum, x 600. 



2. — Side view of an annul us with its projecting plates, x 600. 

 3. — Free end of a male filament, the last 6 frustules being semi-de- 

 tached, x 200. 

 , 4.— Male frustule, X 600. 



5. — Female frustule with 6 male frustules attached : the earliest stage 



of conjugation, X 200. 

 6. — Young single sporangium, X 250. 

 7. — Mature ditto ditto with new frustule, x 300. 



Plate VIII. 



Fig. 1. — Double sporangium shewing different degrees of development, 

 X 200. 

 2. — Mature ditto ditto with 2 new frustules, X 300. 



