156 The Ohio Naturalist. [Vol. XIII, No. 8, 



tological studies of mosaiced tomato, but did not find any char- 

 acteristic abnormalities. No striking difi^erentiation was seen 

 between the yellow and adjoining green or healthy tissues. I 

 did not find stages as described by Woods (1900), where the 

 palisade parenchyma was tmdeveloped or the presence of con- 

 spicuous cuboidal palisade cells as described b}^ Iwanowski (1903) 

 for tobacco. Although at times in the yellow areas this tissue 

 appeared slightly less developed then usual. The yellow areas were 

 slightly thinner than the adjacent green areas, especially in older 

 leaves. The epidermis appeared nonnal. No difference was detected 

 in the number or size of the chloroplasts in the yellow and green 

 areas. That they were well supplied with starch was apparent 

 from the slides and especially in the sections from the older tissue. 

 Potato. — Sections of yellow, adjoining green and healthy 

 tissue of potato mosaic, were fixed in weak chromacetic fluid 

 and imbedded in the usual manner. A microscopic study showed 

 that the yellow areas were thinner at all ages; in some cases they 

 were only 90 mic. thick as compared with 120 mic. in the normal 

 leaf. (See Fig. 1, 2, pi. VIII.) This thinness was largely due to a 

 shortening of the palisade cells which were of a striking cuboidal 

 fomi (Fig. 1, pi. VIII). vSections from mottled areas were easily 

 distinguished by the shape and size of the palisade cells. The 

 cuboidal cells began very abruptly in some sections, while in 

 other cases there was an intergradation between them and the 

 nonnal palisade cells. In the yellow areas as a rule, these cells 

 were generally quite regtilar in shape, but sometimes there was 

 less regularity. Their length varied from one-half to one-third 

 that of normal cells and their thickness was usualh' slightly 

 greater. The spongy parenchyma appeared normal in all areas, 

 except that in the yellow regions, there were somewhat fewer 

 chloroplasts. Figure 3, p\. VIII, represents a green area, adjoining 

 a yellow spot. The palisade cells are slightly shorter than in 

 Fig. 2. The chloroplasts throughout the yellow regions in living 

 material were a pale yellowish-green, but contained considerable 

 .starch. 



Characteristics or Mosaic Disease. 



Infectious. — Investigators who have conducted inoculation 

 experiments with this disease on tobacco find it transmissible 

 by means of the juice. Mayer (ISSO), Sturgis (1S99), Hunger 

 (190.")) and others, have shown that it must be classed as infectious 

 rather than contagious, for the mere presence of a diseased plant 

 in a healthy plot does not cau.sc the disease to sjiread. Numerous 

 investigators have inserted diseased leaf tissue into healthy plants 

 and i:»roduced the disease; in grafting healthy and diseased plants, 

 similar results were obtained, Iwanowski (1903), Woods (1902) 

 and Hunger (1904, 1905). Heintzcl (1900) states, that he got 



