44 
Journal of Agricultural Research 
Vol. XXV, No. a 
amoeboid bodies have been found associated with mosaic of tobacco ( 18 , 
34), with mosaic of sugar cane (26), with mosaic of com (22), with mosaic 
of Hippeasirum equestre (23) , with mosaic of Hippeastrum johnsonii (2 5) , 
and with rosette disease of wheat (24), nevertheless, the causal relation 
of these bodies and organisms to those diseases remains to be demon¬ 
strated. For convenience this unknown cause, which seems to be asso¬ 
ciated with the plant juice, will be referred to here, as elsewhere, as a con- 
tagium or a virus. Degeneration diseases are further characterized gen¬ 
erally by dwarfing and chlorosis, and by the absence of the first symp¬ 
toms from foliage that has attained complete growth before the introduc¬ 
tion of the virus. 
In the absence of definite information on the exact causes of degenera¬ 
tion diseases of plants, it is necessary to define such diseases, both as a 
class and as individuals, entirely in terms of behavior and symptoms. 
A number of characteristics are considered by the writers and others as 
being elementary “unit symptoms” of degeneration diseases of the 
potato. Dwarfing consists essentially of reduction in size of parts 
(PI. 2, A, 2) rather than in their number, although both may occur 
together. The supporting parts of the shoots and leaves are reduced in 
length and thickness. The leaf blades are reduced in area, possibly 
only apparently so sometimes when wrinkling or ruffling is present. 
Spindliness is lateral dwarfing, resulting in stems or tubers being ab¬ 
normally slender (PI. 8, A, 2, C, 1, 2). Chlorosis is a yellowing 01 
paleness that affects the leaf blades and is assumed as being diffused 
unless designated as mottling. Mottling is a localized chlorosis consist¬ 
ing of spotting of the leaf blades by light green areas, which may or 
may not occur in contact with the larger veins, and which vary in shape 
and degree of paleness. These discolored spots are punctate, elongate, 
circular, angular, and irregular. They vary from a barely discernible 
fading of the green to an almost pure yellow, often in the same spot. 
They seldom exceed a few millimeters in any dimension, and their dis¬ 
tinctness of outline differs, usually in proportion to the degree of dis¬ 
coloration. They are more readily seen in diffused light than in direct 
sunlight. Wrinkling is an abnormal unevenness of the leaf-blade surface 
due to depressions and prominences not arranged in any uniform manner 
as with rugosity (PI. 1, A, 1,2). Rugosity differs from wrinkling in hav¬ 
ing depressions only at the veins and in having the prominences of 
uniform height (PI. 3, C; 4, A, 1). Ruffling is an abnormal uneven¬ 
ness of the leaf-blade surface caused by ridges that develop or become 
more pronounced with passage from the midrib to the lateral margins, 
resulting in waviness of the margin (PI. 1, A, 1, 2). Curling is an 
abnormal bending of the leaf blade downward along the main vein 
(PI. 3, A). Rolling is an upward curving of the sides of each leaflet 
with the midrib at the bottom of the trough thus formed (PI. 6, C, [2). 
Uprightness may appear in both leaves and stems but is assumed to^be 
in the latter alone unless otherwise stated (PI. 10, B, 2, C, center). 
It is characteristic of normal plants in most varieties when small and 
probably in diseased plants is often due to dwarfing. Rigidity may 
affect either stems or petioles, or both; brittleness may do the same 
(PI. 6, A, 2, B, 1). Necrosis is the premature death of tissues accom¬ 
panied and manifested by their turning brown. It may appear as spot 
necrosis or spotting (PI. 5, B), with brown flecks of leaf blade usually 
more conspicuous on the upper surface; or, as streak necrosis or streak¬ 
ing, with brown streaks first, most evident on the lower surface of the 
