SYMPTOMS OF DISEASE (SYMPTOMATOLOGY) 343 



12. Exudations. 

 Slime flux. 



Gummosis: especially for stone fruits. 

 Resinosis: especially for coniferous trees. 



13. Rotting: 



Dry rot and soft rot: "the gangrene" of plant tissue. 



Root rots: alfalfa, cotton, beets, cherry, etc., generally woody or 

 fleshy roots. 



Stem or trunk: dry rot of trees; rot of modified stems like rhi- 

 zomes, bulbs, or tubers. 



Buds. 



Fruits: fleshy fruits of various kinds. 



It will be profitable to discuss the symptoms of disease under the 

 above heads. 



I. Discolor ations. — The unnatural, or false color which plants 

 assume under diseased conditions may be included under the head of 

 discolorations. Sometimes, as in woods, the discoloration may appear 

 as a stain. Abnormality of color usually accompanies other symptoms 

 of plant disease. Pallor, or chlorosis, where the plant assumes a yellow- 

 ish to white, or sickly-pale hue, is due to a number of causes. Promi- 

 nently, one form is due to the absence of light, whereby the plant be- 

 comes etiolated, or suffers etiolation. It is considered that the laying of 

 wheat and other cereals is one form of this etiolation where, through 

 lack of carbohydrates, the cellulose which forms the strengthening of the 

 cell wall does not form properly. Sometimes the gardener induces 

 etiolation in his celery, endive and asparagus plants, where the blanch- 

 ing is secured by covering such plants with soil. True' chlorosis is due 

 to an enzyme which destroys the chlorophyll pigment of the chloroplasts 

 which are fully developed. Icterus is the condition where the organs 

 are only yellow; chlorosis, where they are white, such as in the mosaic, 

 or calico disease of plants formerly described. Yellowing may be in- 

 duced experimentally by an excess of carbon dioxide, in fact yellowing 

 accompanies wilting, the attack of wire worms, the presence of poisons, 

 or acid gases. 



Variegation and albinism may be apparently normal conditions of 

 some varieties of plants, for gardeners and horticulturists grow such 

 plants by preference for decorative uses. This variegation, or albinism, 



