Notes. 1 71 
at least in the case of the Bermuda onions it does not appear to be 
the cause of the disease. 
A. L. KEAN, 
Mass. Institute of Technology, Boston, U.S.A. 
A HYBRID DESMXD. — In a gathering made on Dartmoor in 
August, 1889, I met with the first recorded instance of what seems 
to me the phenomenon of hybridism among Desmids. The accom- 
panying figure (Fig. 4) represents the appearance of this organism, 
multiplied 200 diameters. It is clearly intermediate between Euastrum 
crassum , Ktz. and E. humerosum , Ralfs ; the lower half corresponding 
closely to the typical form of the former of these species, the upper 
half to that of the latter. Of the normal forms of these species, 
E. crassum was one of the most abundant in the gathering, E. 
humerosum was only rarely seen. The suggestion 
no doubt presents itself, whether it may not be an 
abnormal form of one of these species. Euastrum 
humerosum is not, according to my observation, nor, 
I think, to those of others, a very variable species. 
E. crassum is, no doubt, subject to considerable 
variation. The var. cornubiense mihi (Journ. R. 
Microsc. Soc. 1887, PI. IV. f. 18), presents a certain 
approach to the high shoulders characteristic of 
E. humerosum ; and in several of De Wildeman’s 
figures in his very careful account of the varieties of 
E. crassum (Observations sur quelques Desmidides, 
Bull.R. Soc.Bot.Belg. 1888), the two semi-cells present notable inequali- 
ties. Several writers have also suggested that E. humerosum may be 
simply a variety of E. crassum. But, assuming that it is most convenient 
to regard the normal forms as distinct species, I think we have here 
considerable evidence of hybridity. The two half-cells are distinctly 
unequal in size, the lower or larger half measuring 90 /x in length, by 
92*5 /x in breadth, the upper or smaller half 82*5 /x in length and 
breadth. The lower half has but a slightly diminishing diameter as 
far as the first and only deep indentation, in fact the normal form of 
E. crassum ; while in the upper half there are, as in E. humerosum, 
first a shallow and then a much deeper indentation, the diameter 
narrowing rapidly towards the apex. It is interesting, however, to 
observe that the arrangement of the protuberances or inflations is 
