172 BOTANY. 
neath the epidermis, which is soon ruptured, exposing the 
spores (Fig. 94, II) in reddish lines or spots upon the leaves 
and stems. This is the Red-rust stage, so common before 
wheat-harvest. These red spores fall easily, and quickly 
germinate, producing more Red-rust (Fig. 95, D), and so 
rapidly increasing the parasite. 
357. (III) Somewhat later in the season the same para- 
sitic filaments which have been producing Red-rust spores 
begin to produce lines or spots of dark-colored, thick-walled, 
two-celled bodies (teleutospores), the so-called spores of 
the Black-rust (Fig. 94, ZZZ). Being thick-walled, 
they endure the winter without injury, and when spring 
comes (IV) they germinate on the rotting straw and pro- 
duce several minute spores, called sporids (Fig. 95, A and 
B). This is the fourth and last stage of the rust. The 
sporids fall upon Barberry-leaves and germinate (Fig. 95, 
C), giving rise to cluster-cups again. 
These stages are so different in appearance that for a long time they 
were regarded as distinct plants, and received different names. Thus 
the first stage was classified as a species of Aicidium, the second as a 
species of Uredo, and the third as a Puccinia. We still preserve 
these names by sometimes calling the spores of the first ecidiospores, 
and of the second uredospores, while the third name is retained as 
the scientific name of the genus. 
The sporids cannot ordinarily produce rust directly upon wheat, 
probably because of the toughness of the epidermis; but it has re- 
cently been discovered that when sporids germinate upon very young 
leaves of wheat-seedlings, they penetrate the epidermis and then soon 
give rise to a red-rust stage. In such cases the cluster-cup stage is 
omitted. This is no doubt the mode of propagation of rust on the 
spring wheat so largely grown in the Mississippi Valley, and also 
upon oats and barley (sown in the spring), which are affected by the 
same species. 
There are many kinds of rusts, distinguished mainly by their te- 
leutnspores, which are one-celled (Uromyces and Melampsora), two- 
celled (Puccinia and Gymnosporangium), or many-celled (Phragmidi- 
