West & West. — Observations on the Conjugatae . 47 
of the contents of the cells, and in these cases the spore is 
generally in the female filament (Fig. 74). Some examples 
have a spore in each conjugating cell (Figs. 77-80), and as 
a rule that in the female cell is of larger size. Occasionally 
the spores are of equal size (Fig. 78), and in rare cases the 
largest spore is in the male (?) cell. 
All this leads up to cross-conjugation, which is the only 
other objection to sexuality. By cross-conjugation we mean 
scalariform conjugation with the formation of perfectly normal 
zygospores in each of the conjugating filaments, and we have 
seen but a solitary example of this amongst the thousands 
of conjugating specimens examined. This was a specimen of 
Spirogyra gracilis (Fig. 81) found in a gathering of Desmids 
obtained from a mass of Utricularia minor in a bog near 
Bowness, Westmoreland. As will be seen from the figure, 
there are two female cells and one male cell in one filament, 
and two male cells and one female in the other ; moreover, the 
zygospores in each filament are perfectly normal, and the con- 
jugation is complete and also normal. Now this is explicable, 
as in the case of lateral conjugation, by supposing that 
each individual cell has assumed sexuality. That the sexual 
condition of the filaments is the same in both lateral and 
cross-conjugation is proved by the occurrence of the former in 
both male and female filaments, which are also conjugating 
in a scalariform manner 1 . 
As a rule examples with zygospores in both filaments only 
exhibit a false cross-conjugation (Fig. 64), the zygospores in 
one filament being smaller than those in the other. This fact 
tends to prove that numerous attempts at cross-conjugation 
result in failures, normal zygospores not being produced, and 
together with its extreme rarity serves to show to what degree 
it is abnormal. 
From the foregoing statements we have shown that lateral 
and cross-conjugation are explicable from a sexual point of 
view, and that there is no reason to regard the Zygnemeae as 
otherwise than sexual. 
1 Petit, in Bull. Soc. Botan. France, fevr. 1874, t. xxi, PI. I, f. 2. 
