SCpt. 33, 1923 
Effect of Orange-Rusts on Stomata 
499 
occurred only on a part of the leaf (PI. 1, A). Aecidiospores were sowed 
over several plants in such a way that most of the spores fell on the 
dorsal side of leaves which had fully expanded. Many spores must have 
come in contact with the underside. The leaves on the tips of young 
basal shoots which had not unfolded were tagged. Particularly large 
numbers of spores were sowed on these folded leaves. The experiment 
was repeated April 25 with other plants. , On May 25 teleutosori in 
abundance were found on plants from both sowings, showing that it takes 
at least four or five weeks in the greenhouse for the teleutosporic stage 
to reach maturity. Some of the leaves now bearing teleutosori also bore 
aecidia and spermogonia, while others bore only spermogonia; teleuto- 
spores were more abundant on such leaves. 
In case aecidia or spermogonia occurred on only a part of a leaf, it 
was found that teleutosori were present only on that part, often on the 
upper as well as on the lower side. The part of the leaf not invaded by 
the gametophytic mycelium and free from the orange-rust stage had 
escaped infection when aecidiospores had been sowed on its dorsal sur¬ 
face. Leaves bearing teleutosori were decolorized and examined for 
stomata. The normal number appeared to be present on the ventral side 
of those leaves bearing only spermogonia and teleutosori. It was, of 
course, impossible to determine what had been the distribution of stomata 
on those areas on the ventral side of leaves where aecidia had destroyed 
the epidermis. Stomata were always found in abundance wherever teleu- 
tospores were present. Where aecidiospores of the Gymnoconia had been 
sprayed on both sides of leaves of old canes systemically infected, teleuto¬ 
sori first appeared on parts of leaflets having spermogonia. About 10 
days later they began to appear among the hairs on the under side of 
neighboring leaves that had escaped invasion by the orange-rust hyphae. 
The results point clearly to the value of stomata on the upper side of 
leaves in facilitating the attack by the sporophytic germ tube. The 
gametophytic mycelium of the rust stimulates the host to provide ready 
means of access by the sporophytic stage which is to follow later. 
As previously noted, the leaves of the black raspberry which are exposed 
to infection as the aecidiospores are being shed, may be grouped as follows: 
1. Fully expanded but somewhat dwarfed leaves on old canes already infected 
with the systemic or orange-rust stage. The leaves or parts of leaves into 
which the gametophytic hyphae have penetrated will be rather devoid 
of hairs on the lower surface and have an abundance of stomata on the 
dorsal side, two factors favoring infection by the germ tubes. 
2. Large, fully unfolded leaves on infected basal shoots from systemically 
infected plants such as produce the old canes in No. 1. Gametophytic 
hyphae evenly distributed throughout most of the leaflets, which will 
later usually be covered with aecidia. Very few hairs on the lower side, 
many stomata on the dorsal side. 
3. Fully expanded leaves on normal old canes. Leaves somewhat tomentose 
on the underside. No stomata on the dorsal side (a very few at the tips of 
the serrations). 
4. Very young leaves at the tips of systemically infected basal shoots. Some 
of these leaves will be tightly folded, others just expanding. 
5. Fully expanded and growing leaves, tomentose beneath, on normal basal 
shoots. 
6. Very young leaves, some still folded and very tomentose, others expanding, 
on basal shoots of normal canes. 
The infection experiments described above show that some of the 
factors which determine the readiness with which a leaf of the raspberry 
can be infected by sowing aecidiospores resulting in the development of 
the teleutosori (Puccinia peckiana) are: Presence or absence of stomata 
