SCIENCE 



NEW YOEK, DECEMBEE 15. 1893. 

 DROPSICAL DISEASES OF PLANTS. 



BY GEO. F. ATKINSON, BOTANICAL DEPARTMENT, CORNELL UNI- 

 VERSITY. 



Eecent progress in the study of fungi and bacteria has 

 revealed many standing in casual relation to several plant 

 diseases which were of a hitherto mysterious nature and 

 which were supposed to have been due to atmospheric or 

 solar influences. The efifect has been to create a firm be- 

 lief in the germ theory of plant diseases, and there is 

 consequently a strong tendency in certain quarters to ap- 

 ply the germ theory to all cases of phytopathology. The 

 first fruits of investigations leading to the determination 

 of bacteria and fungi as the cause of certain obscure 

 blights of plants were either rejected in toto, or received 

 cunt grano salis. Now it is the fad to attribute all phases 

 of disturbance, short of artificial injuries or those violent 

 injuries produced by the elements, to the agencies of 

 micro-organisms. 



It must be urged, however, that there are some purely 

 physiological diseases of plants. But so strong just now 

 is the leaning to the germ theory that any departure 

 from this path must be fortified by very careful experi- 

 ments and inoculations with the germs associated with 

 the trouble in order to carry conviction not only to one's 

 self but to others. 



The writer had occasion last winter to investigate a 

 very mysterious disease of tomatoes' grown in the forc- 

 ing houses of Cornell University. Typical cases of the 

 disease presented the appearance so characteristic of cer- 

 tain mildews. The leaves were strongly curled and the 

 veins on the under side were swollen and whitened. 

 Large patches of the same kind were also found to exist 

 on the stems. Contrary to expectation, a microscopic ex- 

 amination did not show the j)resence of any fungus of or- 

 dinary dimensions in the early stages of the trouble. The 

 young and succulent tissues of the plant were strongly 

 turgescent and the cells in the affected areas were 

 stretched radially to an enormous extent. These tissues 

 ultimately collapsed and in many instances the loss of 

 water content from these places was so great that the 

 affected parts of the plants died. In other cases the loss 

 of water was more gradual where the turgescence and 

 rupture of the cells were not so profound. Then the cush- 

 ions of collapsed tissue were dry, and presented a tomen- 

 tose appearance, and the effect upon the adjacent tissues 

 was to interfere with the assimilatory processes, causing 

 the upper parts of the leaves at these spots to become 

 yellowish in color. On the stems there resulted from the 

 collapse of the tissues elongated, depressed and black- 

 ened areas in various stages of decomposition. These 

 sometimes extended far beyond the first appearance of 

 the cushions of elongated cells, showing that the trouble 

 once started would extend to other parts of the plant. 



The question now to be considered was. Are these ab- 

 normal extensions of tissue caused by the influence of 

 some microsymbion within the plant ? Healthy plants 

 were inoculated with the material taken directly from 

 affected tissues. No result. 



Dilution cultures were then made from several different 

 affected places and fifteen different species of bacteria 

 were obtained, three of BaciUuf!, three of Micrococcus, and 

 nine of Bacterium. Cultures of these were made in liquid 



'Oedema of the tomato, Bull. 53, C. \j. Exp. Sta., May, 1893. 



media and healthy plants were inoculated, but there was 

 no result. 



These negative results from inoculation and also the 

 fact that when using the necessary precautions to prevent 

 the entrance of gems from the outside, no growth oc- 

 curred when culture media were inoculated with material 

 from the interior of freshly affected areas of the plant, 

 led to the conclusion that the trouble was due to some 

 physiological disturbance of the plant, probably that root 

 pressure, as it is termed, was greater than transpiration. 



The conditions of the green house were such as to pro- 

 duce active and almost constant root absorption, while 

 they were very unfavorable for transpiration, since there 

 was very little disturbance of the air, the jaart of the 

 house where the tomatoes were grown was poorly light- 

 ed, and the winter days were very short as compared with 

 the nights. In the ojien during the summer currents of 

 air remove quickly the water vapor given off during 

 transpiration. This, with the longer days, favors rapid 

 transpiration. The heating of the forcing house is also 

 such as to make but little difference between the temper- 

 ature of the soil and that of the air, and also the temper- 

 ature of the soil is such as to make the roots active al- 

 most continuall}', while in the open there is a much 

 greater difference between the temperature of the soil 

 and air, in such ratio in the summer that root pressure 

 and transpiration are more evenly balanced. 



Cuttings of healthy plants were then connected with 

 the hydrant by means of rubber tubing, the pressure of 

 water turned on being twenty to thirty pounds The 

 pressure of water was so great that in a very few minutes 

 drops of water stood out at the ends of the veins on the 

 margins of the leaves. In a few days, since this abnormal 

 pressure was in excess of transpiration, cushions of 

 turgescent tissue exactly like those developed under the 

 conditions of the forcing huse, were produced. 



Another proof that the cause of the trouble was exces- 

 sive turgescence is furnished by the relation of growth to 

 the trouble. Where active growth was taking place in 

 the cells the radial elongation did not take place. The 

 increase in number of cells and the natural increase of 

 the size of the cells were sufficient to accommodate the 

 amount of water distributed to those parts of the plant. 

 So that when there was no immediate interference with 

 the growth of the terminal portions of the plants there 

 were no cushions developed on the ends of the stem or 

 branches for a distance of four to six inches. But just so 

 soon as the growth by increase of cells ceased the cush- 

 ions of turgescent tissue appeared. Also in the case of 

 some plants on one bench, when the tops reached the 

 glass roofing and the confined condition interfered with 

 growth, the trouble appeared even to the extreme tips of 

 the stem and branches. It is also to be noted that the 

 short days and poor lighting of the house gave little 

 oj)portunit3' for the metabolic processes in the manufacture 

 of building material for the formation of strong cell 

 walls. 



Recently a similar trouble has fallen to my lot to in- 

 vestigate, which occurred on apple trees. Numerous 

 blisters ajjpeared on the trunk and branches of young 

 and vigorous orchard trees. These blisters were caused 

 by the radial elongatian of the phellogen layer of cells 

 just beneath the periderm. The tissues ultimately col- 

 lapsed, and were then subjected to the attacks of putre- 

 factive bacteria and fungi. Inquiry of the owner devel- 

 oped a fact which was already inferred, that the plants 

 were under such conditions that growth was very rapid, 

 and then during the winter and spring were subjected tt> 



