8 BULLETIN" 1038, U. S. DEPARTMENT OF AGRICULTURE. 
in natural and artificial infections. Diastase activity in the healthy 
leaves was found to be almost double that in the spiked leaves. The 
mesophyll tissues are hypertrophied and the leaves increased in 
thickness and the vascular tissues become reduced in advanced stages 
of the disease. Chemical analysis (24) shows a higher percentage of 
nitrogen and of most of the ash constituents in healthy leaves. Since 
young, healthy leaves pass through a stage comparable in chemical 
composition to that of spiked leaves, it seems probable that in the 
latter case development has for some reason been checked in the early 
stages of growth. 
In a large percentage of Coleman's grafting experiments (24) 
spike was successfully transmitted to originally healthy stocks, and 
in almost every case the disease first appeared in the stock on branches 
closest to the point of grafting and spread from these regions to the 
other parts of the stock. A considerable time always elapsed before 
external signs of disease appeared. In all cases examined, infection 
had spread to the roots and resulted in the death of the root ends 
and haustoria. 
The occurrence of spike disease is not dependent on the fertility 
of the soil, nor does injury to the roots have any relation to the dis- 
ease. Venkatarama (81) found experimentally that isolation of the 
trees from all possible hosts by digging trenches did not cause signs 
of spike even after two years under observation. Cutting the root 
connections and removing the haustoria, injecting the lateral roots 
with strong sulphuric acid, and girdling to the heartwood did not 
cause the disease. Thousands of trees previously growing under a 
heavy covering of vines were exposed to the light, but the increased 
loss of water resulted in nothing resembling spike disease. Fischer 
(34) states that the spike disease spreads from a center, not appear- 
ing simultaneously over considerable areas. 
In tobacco mosaic the yellow and green mottling of the leaves is a 
prominent sign. Not only leaves but also the calyx may be mottled, 
and the corolla becomes flecked with red and white blotches instead 
of exhibiting the normal even red or white color. The light areas of 
the leaves are usually slower growing than the green areas, thus often 
resulting in distortions which may become extremely marked in 
young leaves. Often, however, such leaves almost recover from these 
malformations as they mature (1). Sometimes the laminae are al- 
most suppressed, and in other cases a long, sinuous, ribbonlike leaf 
is produced. In many cases abnormally dark-green blisters develop 
on the immature leaves. 
Koning (47) and Heintzel (40) reported a separation of the cells 
in diseased foliage which often leaves spaces nearly as large again as 
the cell itself. The chlorophyll bodies become distributed in irregular 
groups, chlorophyll disappears, the cell walls disintegrate, and finally 
