167 



I would remark that the Zygnema quadratum is a species with the 

 ends of the cells inverted, and in which conjugation takes place be- 

 tween contiguous cells of the same filament. 



In the Zygnema varians the conjugation, on the contrary, occurs 

 between the cells of different filaments, and the ends of the cells are 

 not inverted. In this species, also, I have found the stellate sporan- 

 gia, though as yet not quite so plentifully as in the preceding. There 

 is a slight difference in the appearance of the two sporangia, the 

 spines being more acute and lengthened in the latter ; but the dis- 

 tinction is not very marked to a casual observer. I have also a few 

 specimens of a stellate sporangium differing materially from the two 

 preceding, which I conceive to belong to a species of Tyndaridea 

 (though this is little more than conjecture), in which the spines are 

 infinitely more numerous and delicate, and bear a close resemblance 

 to the figure of what is called Xanthidium hirsutum in the first vo- 

 lume of the ' Transactions ' of this Society. (Plate XXII. fig. 3). 



I have found the new gelatinous medium recently suggested by 

 Messrs. Wenham and Deane extremely convenient for mounting the 

 sporangia as permanent objects, though it does not answer for mount- 

 ing the AlgcB before conjugation ; and I purpose depositing in the 

 cabinet of the Society specimens thus prepared of the objects I have 

 so inadequately attempted to describe. 



It is not very surprising that these interesting bodies should have 

 been pretty generally overlooked, when we remember that as soon 

 as the union of the conjugating cells has been completed the Alg<B 

 generally sink to the bottom of the water ; consequently, if any one 

 should feel sufficiently attracted to the subject to enter into any more 

 lengthened investigations than I have been able to accomplish, I 

 would by all means recommend him to secure such specimens as 

 he may find in fruit, in any stage, and, having placed them in glass 

 jars, expose them to the influence of the sun, but in such a way that 

 they can, by falling to the bottom, be protected from it. 



In conclusion, I would remark, that the gradual change of form in 

 the sporangia of the filamentous Alga is precisely analogous to the 

 progressive increase of the spines of many of those belonging to the 

 allied Desmidiece ; and, with regard to the designation of these bodies 

 as sporangia, I have simply adopted that made use of by those best 

 acquainted with aquatic botany, without attempting, from any obser- 

 vations now made, to adduce any reason for deciding whether they 

 are truly sporangia or only spores. 



