OEDEMA 
_ This disease, although seldom of much importance under field condi- 
tions, sometimes becomes destructive in the greenhouse. It is of interest 
to the plant pathologist in that it is not caused by an organism, but may 
readily be induced by changes in temperature and humidity or by the 
application of dilute solutions of certain toxic substances. It affects many 
plants but especially tomatoes and cabbage. 
SYMPTOMS 
The cabbage leaves provided were gathered from plants in a vegetable 
garden. The seedlings had been grown in the greenhouse, and the plants 
set out in the spring. The season was cold and rainy. OBSERVE:— 
1. That all the lesions are on the under side of the leaf. Is there 
any special reason why they should not be on the upper surface? 
2. That these lesions are small raised spots, and are roughened 
like scab-spots on potatoes. Do they appear on the veins or on the areas 
between the veins? Is there any definite shape to the intumescences? 
Make a DRAWING of a portion of a diseased leaf. 
Study and skETcH the symptoms as exhibited by potato, tomato or 
other plants provided. 
ETIOLOGY 
No organism is connected with this disease. It is caused by conditions 
which produce abnormal turgidity in the spongy parenchyma-cells of any 
part of the plant above the ground. It has been induced artificially by 
attaching the cut end of the stem to a hydrant where the water-pressure 
was very strong. The same result has also been accomplished by spray- 
ing the leaves with-a dilute solution of ammoniacal copper carbonate. 
The larger drops always kill the tissue outright, but the smaller particles 
of spray cause an abnormal increase of the parenchymal cells below the 
epidermis. In the greenhouse, the disease results when the soil is warm, 
so that the roots take in a large amount of water, and the atmosphere 
is cold and the place poorly lighted, so that transpiration does not take 
place as fast as absorbtion. This causes such a pressure in the thin-walled 
tissues that hypertrophy and rupture result. 
Under direction of the instructor, the students may undertake some 
experiments in the greenhouse to demonstrate the causal relation of some 
of the factors just mentioned. : 
Pathological Histology. Cut thin cross-sections through a lesion or 
use prepared slides. OBSERVE:— 
3. That the lesion is made up of a number of abnormally large 
cells, the ends of which are entirely exposed. ; 
P 4. That this exposure is due to the rupturing and the breaking- 
away of the epidermal cells. Are fragments of these still remaining: 
5. That all the hypertrophied cells are those of the parenchyma. 
6. Changes in the organs of affected cells. ah 
Make prawincs to show comparatively the conditions in diseased and 
healthy tissues. : 
REPORT 
1. Describe in detail the procedure and results in the etiologic 
experiments, conducted under the instructor’s directions. 
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