38 DISEASES OF CULTIVATED PLANTS 



passed through the immune branch. This passing of the 

 virus unaffected through a portion of an immune plant does 

 not always hold good. If a scion of Lavatera arborea is 

 grafted on A. Thomsoni, and another susceptible portion is in 

 turn grafted on the L. arborea portion, the leaves do not 

 become variegated. In this instance the virus loses its 

 potency in passing into the intermediate immune scion of 

 L. arborea. The author considers that this form of varie- 

 gation or chlorosis is due to the presence of a virus depending 

 on light for its formation. When grown in the shade suscep- 

 tible plants lose the variation and become green, although the 

 general health of the plant is not affected. Experiments 

 proved that the virus travelled in the cortex and not in the 

 wood. 



Baur, E., Ber. d. deutsch. Bot. Geselts., 24, p. 416 (1906). 



INJURY BY SMOKE, ACID FUMES, GAS, ETC. 



Sulphur dioxide has been proved to be the specific cause 

 of injury to vegetation arising from smoke. Wieler, who has 

 recently investigated the subject in an exhaustive manner, 

 states that, contrary to the view of von Schroeder and Rensz, 

 sulphur dioxide, like other gases, enters into leaves only 

 through the stomata. Leaves having the surface-bearing 

 stomata coated over showed no injury, when exposed for 

 several hours to a comparatively strong concentration of 

 the gas. 



In very young leaves the gas penetrates the cuticle. 



The physiological effect of sulphur dioxide on foliage is 

 very complicated. Probably its acid nature and its capacity 

 for forming certain products with aldehydes present in the 

 tissues act on the protoplasm. Wieler recognises two kinds 

 of injury, acute and chronic. The acute condition is rare, 

 and is manifested only in the immediate neighbourhood of 

 the source of the smoke where the quantity of gas is sufficient 

 to kill the tissues directly. In chronic cases, where the gas 

 is present only in a very small percentage, respiration becomes 

 irregular, photosynthesis is partly checked, probably due to 

 the direct action of the gas on the chlorophyll rather than to 

 the closing of the stomata. The removal of the products of 

 photosynthesis is somewhat checked, and growth is retarded. 



