475 
THE WHITE RUST, ETC. 
that when these tubes have entered the stomata of the coty- 
ledons they branch and ramify, becoming a true mycelium, 
from which fruitful parasites are developed ; that if a plant 
so infested lives through the winter the parasite lives with it 
to vegetate again in the spring. 
The immense number of zoospores capable of being pro- 
duced from a single infested plant is almost beyond cal- 
culation. It is easy for a million of conidia to be developed 
from such a plant, each producing from five to eight zoospores, 
besides a large number of oospores each containing a hundred 
zoospores. It can scarcely be considered marvellous that the 
white rust should be so common on plants favourable to their de- 
velopment, the marvel being rather that any plant should escape. 
Until recently it was doubtful whether more than one or 
two species of Gystopus (white rust) were known. It is now 
certain that we have two in Great Britain, and five or six 
others are found elsewhere. Of the British species one is 
found on many cruciferous plants, as the shepherds purse, 
garden-cress, mustard, radish, and plants of the cabbage kind. 
This is the Gystopus candidus. Another occurs on the goaffs- 
beard, salsify, and scorzonera, which is called Cyst-opus cubicus. 
Both have great external resemblances, but both possess 
specific internal differences. In the goaffs-beard rust the 
terminal conidia in the bunches or fascicles of conidia which 
are produced within the pustules are spheroidal, large, and of 
a yellow-brown tint, whilst the residue are cylindrical, smaller, 
and more or less compressed. In the crucifer rust the conidia 
are ail equal in the pustules and globose. The oospores in 
the former of these are subglobose, and the warts on their 
surface are solid ; whilst in the latter the oospores are truly 
globose, and the warts on the surface are hollow. Of the 
other species it is not improbable that one or two, perhaps 
three, may yet be found in this country. Without attempt- 
ing to indicate their microscopic differences, it may be 
serviceable to name the species of phanerogamic plants on 
which they are likely to be found. The purslane white rust 
(' Cystojpus Portidacce , D. C.) should be sought on the purslane, 
which, though of limited cultivation, is exceedingly liable to 
attack from this parasite. The sandspurry white rust {Gy st opus 
Lepigoni , D. By.) may perhaps be found on the common 
sandspurry * (Spergularia rubra) ; and the thistle white rust 
{Gystopus spinulosus, D. By.) may probably be met with on 
the leaves of the common thistle {Gnicus arvensis ) or some of 
its allies. 
Considerable interest is now attached to these parasites, 
'* Whilst this paper was passing through the press, I saw British specimens 
of this species also collected as a variety of Cyst opus candidus last autumn. 
