West & West. — Observations on the Conjugatae . 53 
and Zygnema , although, as in most filamentous Desmidieae, the 
filaments break up before conjugation. So far as we know, this 
is the only case of differentiation of the conjugating cells 
met with in the whole of the Desmidiaceae. Boldt 1 figures a 
‘ forma monstrosa 5 of Hycilotheca dissiliens with the zygospore 
in one of the cells, and Joshua 2 mentions a case where Hyalo- 
theca dissiliens was conjugated like Desmidmm cylindricum . 
Fig. 37 is also an approximation to this stage in an example 
of Hyalotheca dissiliens. What is this c monstrous form 5 of 
conjugation in this species? Abnormal it certainly is as 
compared with ordinary conjugated examples, but is it not 
a case of reversion to some ancestral type of conjugation, 
represented at present by the Zygnemeae, and which the 
Desmidieae have almost lost, the lingering remains of which 
are still found in Desmidium cylindricum ? 
Thus degeneration and loss of sexual differentiation of the 
conjugating cells have gone on hand in hand with the loss of 
the filamentous condition, the majority of filamentous forms 
dissociating before conjugation. An extreme morphological 
specialization has accompanied this loss of the filamentous 
condition, causing the large majority of this family of 
unicellular plants to be remarkable for their beauty and 
variety of form. 
In the genus Desmidium conjugating-tubes are formed, and 
we have noticed rudimentary conjugating-tubes in some 
species of Closterium z and in Arthrodesmus octocornis. 
Conjugation seems to take place in many Desmids 
immediately after division and before the young semi-cells 
have had time to attain maturity 4 : for sexuality to exist 
West and G. S. West, N. Amer. Desm., Trans. Linn. Soc. Bot., ser. 2, Vol. v, 
Pt. v, PI. XII, f. 29. 
1 R. Boldt, Desm. fran Gronl., Bih. till Sv. Vet.-Akad. Handl., Bd. xiii, Afd. 3, 
No. 5, T. II, f. 33. 
2 W. Joshua, Notes on Brit. Desm., Journ. Bot., Vol. xx (1882). 
3 In Closterium Ehrenbergii they are perforated protuberances at the base of the 
younger semi-cells; cf. West and G. S. West, Journ. Roy. Micr. Soc., 1896, p. 151. 
4 West and G. S. West, 1 . c., pp. 151 and 153, PI. Ill, f. 29 ; also W. Archer, in 
Quart. Journ. Micr. Soc., Vol. ii, p. 251. 
