NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 569 



by Wolf and Lloyd affecting the leaves of rubber-producing plants be- 

 longing to the genus Manikot of which M. glaziovii, M. keptaphylla 

 and M. pianhyensis are known as ceara. The leaves of the ceara 

 plants growing in the greenhouses of the Agricultural Experiment 

 Station, Auburn, Alabama, were found with numerous, glistening, 

 prominently projecting elevations on either surface of the leaf. When 

 the elevations or swellings occur on the upper surface there are corre- 

 sponding depressions or concavities on the lower reaching as much as 

 three millimeters in diameter and protruding a millimeter above the 

 surface. The blisters are circular in outline and mostly isolated, but 

 if they exceed 300 to 500 they become more or less confluent. At first 

 there is no change in the color of the leaves, but as the disease 

 progresses the oedematous tissue turns brown and finally dries and 

 collapses. The anatomic details of healthy as contrasted with the 

 diseased oedematous cells are shown in the accompanying details of 

 Figure 206. 



A number of explanations have been given for the origin of cedema, 

 or dropsy in plants. Giant cells have been found in dropsical tissues 

 similar to those found in insect galls. Woods found that thin walled 

 oedematous cells were found in carnations as a result of the puncture by 

 aphids, and in such the possible acid conditions must be considered. 

 Sorauer and also von Schrenk have shown that intumescences may be 

 caused by spraying leaves with copper salts. Several other plant 

 pathologists hold to the general view that the disease is due to impaired 

 transpiration. Sorauer was the first to attribute the cause to abnormal 

 elevation of temperature, together with excessive water supply. He 

 finds that weak light or semi-darkness favors the accumulation of water 

 in the tissues, in that reduced illumination lowers assimilatory activity, 

 and swollen tissue results. Viala and PacoUet beUeve that briUiant 

 light is a prepotent cause, while Fisher argues that oedema is due to 

 the increased affinity of the colloids of the tissues for water. This may 

 be due to the accumulation of acids and Wolf and Lloyd' believe that 

 the oedematous tissue of ceara seems to afford some evidence for the 

 truth of this contention. 



Frost Necrosis of Potato Tubers. — Jones and Bailey^ have called atten- 

 ' Wolf, Frederick, A. and Lloyd, Francis E.: Qidema on Manihot, Phy- 

 topathology 2: 131-134, pi. 1, 1912. 



2 Jones, L. R. and Bailey, Ernest: Frost Necrosis of Potato Tubers, Phyto- 

 pathology 7: 71-72, Feb., 1917. 



