496 
Journal of Agricultural Research 
Vol. XXV, No. i* 
veins; in every such case the number is so small as to be of little conse¬ 
quence in the economy of the normal plant. It is quite otherwise in 
case of plants systemically infected with either of the orange-rusts. 
Sections of leaves collected at random for another purpose from a 
systemically infected blackberry in New Hampshire show fully twice as 
many stomata on the upper side as they do on the lower side. An 
examination of sections of black raspberry leaves which had been pre¬ 
pared for a study of young aecidia revealed an abundance of stomata 
on the upper side. Further investigations disclosed the fact that the 
systemic stage of these orange rusts regularly so affects the host as to 
lead to the development of a large number of stomata on the upper side 
of the leaf where normally there would have been only a very few or 
none at all. One frequently sees that in a blackberry which has harbored 
the rust for two years or more most of the leaves on the old canes infected 
are evenly covered with spermogonia from the time they begin to unfold. 
From one-fourth to one-half of the total number of stomata on such leaves 
will be found on the upper side. Certain specimens of infected mountain 
blackberry did not show such a large percentage of stomata on the dorsal 
side; the increase in the number due to the stimulus of the rust was very 
marked. The development of spermogonia on leaves of primarily 
infected blackberries, to be noted later, is frequently inhibited, aecidia 
alone being formed. Neverthless, by noting the areas where stomata 
occur in large numbers on the upper side one can tell several days in 
advance just where aecidia are to be developed. Whenever the rust 
at maturity covers only a part of the leaf it will be found that only that 
part is provided with additional stomata on the upper side. As long 
as the epidermis is in a plastic condition the advance of the mycelium into 
new areas is accompanied by the development of additional stomata. 
The effect of the parasite on the production of stomata varies with the 
course of development of the rust. Under certain conditions, spermo¬ 
gonia may be distributed uniformly over the leaf from the first, but the 
formation of aecidia is long delayed or is entirely omitted. There will 
be about the same number of stomata on both sides of the leaf, the 
normal number occurring on the under side. It has also been found that 
when certain blackberries show the rust for the first time after primary 
infection by sowing sporidia of the short-cycled rust, the spermogonial 
stage is entirely lacking. The infected leaves show by their yellowish- 
green margins the extent to which the rust hyphae have penetrated. 
Aecidia will later develop along the discolored margins, then gradually 
spread toward the midrib, preceded by the fading out of the natural 
green and the production of stomata on the upper side. Very few 
spermogonia were found on some 50 plants of the varieties Kittatinny 
and Iceberg the first two years following infection; aecidia developed 
normally. Other varieties, Ancient Briton, Blowers, etc., developed 
spermogonia under similar conditions. The complete suppression of 
this stage in the life cycle of a rust is said to be of rare occurrence. A 
further study of the rust on different varieties of blackberry may serve to 
suggest a reason for the incomplete development or nonproduction of 
spermogonia by other rusts such as the one on the mallows. 
In rare cases aecidia also fail to appear, most of the leaf merely becom¬ 
ing yellowish. Such leaves, though never bearing any spore forms, have 
been invaded by mycelium, as shown by the large number of stomata on 
the discolored areas of the upper side, the usual number appearing 
below. 
