June, 1913.] The Mosaic Disease of the Toynato. 155 



Histology. 



Koning (1S99 [bj, 1900) made histological studies of mosaiced 

 leaves, but says that little is brought to Hght by microscopic 

 examinations. Intercellular cavities occur between the paH- 

 sade and spongy parenchyma of young and old tissue. In 

 some cases he found the chloroplasts disorganized and cell walls 

 disappearing. Bouygues (1903) reported the absence of the 

 epidennis. In old spots the cell contents had disappeared. 

 Woods (1900, p. 17) found that, "a study of the histology of the 

 diseased leaves has now revealed a histological difference which 

 makes it very clear that the light colored areas are not normal 

 and that this difference consists in the fact that in badly diseased 

 plants the palisade parenchyma of the light colored areas is not 

 developed at all. All of the tissue between the upper and lower 

 epidermis consists of a spongy or respiratory parenchyma rather 

 more closely packed than normal. In moderately diseased plants 

 the palisade parenchyma of the light area is greatly modified. 

 Normally the palisade parenchyma cells of a healthy plant are 

 from four to six times as long as broad. In a moderately dis- 

 eased plant, however, the cells are nearly as broad as they are 

 long, or at most not more than twice as long as broad. As a 

 rule the modified cells of the leaf pass abruptly into the normal 

 cells of the green area. " He also found that the light colored 

 areas in both tomato and tobacco contained more than the nomial 

 amount of starch. Heintzel (1900), does not mention any pecu- 

 liarities in the palisade cells themselves, but observed the most 

 striking differences in the intercellular spaces between the palisade 

 cells and the spongy parenchyma of younger and older tissue. 

 These intercellular spaces occur in the dark green, bloated regions, 

 the older tissue having the larger spaces. He believed, these 

 spaces were filled with gas, because their dark color disappeared 

 when they were put in alcohol. The chloroplasts were con- 

 gregated irregularly in small groups. Iwanowski (1903) states 

 that the green areas bordering the yellow are 'abnormally healthy' 

 and that such regions show a vigorous development of all cellular 

 tissue. The yellow areas on the other hand, are thinner and the 

 palisade cells are not so well developed, being very much shortened 

 and cuboidal in form. He speaks of intercellular spaces in the 

 yellow areas. The chloroplasts in these areas are yellowish and 

 while these regions are young, scarcely react to the starch test, 

 but eventually all the chloroplasts come to contain as much starch 

 as they can hold. 



Tomato. — Westerdijk (1910) says that a microscopic examina- 

 tion of mosaiced toinato leaves show nothing worthy of mention. 

 In the yellow areas the chloroplasts are yellowish and slightly 

 smaller and have but little starch. The writer also made his- 



