32 CIRCULAR NO, 120, BUREAU OF PLANT INDUSTRY. 
leaf as it lies folded in the bud. The only suggestion for explaining 
the very irregular manner in which the cells are killed is that some 
of them may be unable to complete their divisions and nuclear read- - 
justments during the night and may thus be left in an unusually 
susceptible condition. Sections of injured leaves prepared by Dr. 
Albert Mann, of the Bureau of Plant Industry, show that nuclear 
and protoplasmic disintegration are the earliest symptoms. The 
damage often begins with the death of a single cell, which results, 
o&course, in increased exposure for-the neighboring cells. 
Plants protected by partial shade suffer less than those that are 
fully exposed, but, on the other hand, full exposure does not induce 
leaf-cut when the plants are growing on wet land, where the surface 
remains moist and is kept cool by evaporation. The moist atmos- 
phere and partial shade afforded by ordinary greenhouse conditions 
also afford complete protection from leaf-cut. 
Even in parts of the same field there may be obvious differences in 
the extent of leaf-cut injury. Plants that stand close together often 
show much less injury than more scattering plants in the same rows. 
Where the soil is too dry to germinate all the seed the leaf-cut injuries 
are more extensive. Such differences indicate the possibility of 
avoiding or reducing the damage from leaf-cut by giving better 
attention to the seed bed and to methods and times of planting and 
thinning. 
ABORTION OF TERMINAL AND AXILLARY BUDS. 
Though mutilation of the leaves is the most frequent and familiar 
symptom of the leaf-cut disorder, abortion of terminal buds is a 
more serious injury. In severe cases of leaf-cut, from 30 to 60 per 
cent of the plants have been found with their terminal buds aborted. 
When the leaf-cut injuries are confined to the individual leaves the 
effect is merely to retard the growth of the plant, but when the termi- 
nal bud is lost the plants are permanently deformed and usually 
produce a much smaller and later crop than normal individuals in 
the same field. 
In the most severe form of the disorder the young seedlings lose 
the buds in the axils of the cotyledons as well as the terminal bud. 
Such plants are unable to form any true leaves, but the cotyledons 
increase in size and the hypocotyl becomes much thickened. In some 
eases the root begins to form a subterranean shoot, like those that 
develop vegetative buds when plants have been killed to the ground 
in the winter. When abortion of the bud takes place higher up, so 
that the plants have one or two true leaves, the blades grow much 
larger than usual and the petioles become greatly elongated. If 
thinning be deferred until the normal plants are 10 inches or a foot 
[Cir. 120] 
