INTERNAL CAUSES OJ? DISEASE 327 



It. is infectious, and to a certain extent contagious. As calico is an 

 important disease of tobacco and tomato a description of it in these 

 plants will serve to show what enzyme diseases are like in general. 

 The leaves present a mottled appearance, being divided into smaller, or 

 larger, areas of light-green and dark-green patches. In the tomato, the 

 light-green areas become yellowish, as the disease progresses, and in 

 very badly affected plants become finally purplish-red in color. The 

 leaves are much distorted, stiff, and badly curled. It attacks other 

 plants, notably the poke weed. Phytolacca decandra, ragweed. Am- 

 brosia artemisicsfolia, Jamestown weed. Datura stramonium. It is 

 probable that peach "yeUows," aster "yellows" are more or less similar 

 to the true "mosaic." Calico is primarily a disease of the green color- 

 ing matter (chlorophyll) of the infected plants; hence it disturbs the 

 normal nutrition of the plant. To this destruction of the chlorophyll 

 the name of chlorosis has been given and calico is, therefore, a state of 

 chlorosis. The contagious nature of calico is shown by experiments 

 which prove that it can be communicated at least in some cases by 

 mere contact of calicoed plants with the healthy. Juice on the hands 

 from calicoed plants when handling disease-free plants will spread the 

 disease in nearly all cases, and this infection is due to the chlorotic juice 

 on the hands of the experimenter. Chlorosis, or calico, usually takes 

 ten to fourteen days to make its appearance after infection and a plant 

 once infected remains permanently so, and all new growth usually 

 becomes calicoed. Calico, or mosaic, can be transferred to other species 

 and varities of Nicotiana than the common N. iabacum, also to potato, 

 egg plant, peppers, petunia, etc. The dried leaves of calicoed tobacco 

 retain their power of infection for at least a year or two, to some degree, 

 but if wetted they lose this power. The virus, if it is permissible to 

 use this word, can be apparently extracted from calicoed leaves by 

 ether, chloroform and alcohol without destroying its infectious qualities. 

 Bunzel has measured the oxidase content of plant juices, because of the 

 importance of oxidase in chlorotic diseases of plants, in their causal 

 relationship to color production in plants, their importance in the dark- 

 ening of tea and in the production of the smooth, black and hard 

 lacquer of the Japanese, from the white, fluid, soft secretion of the 

 lacquer tree, Rhus vernicifera. The literature on oxidizing enzymes 

 is a copious one. The following papers and books can be consulted, 

 as well as the bibliography which each includes: 



