32 CIRCULAR NO. 120, BUREAU OF PLANT INDUSTRY. 



leaf as it lies folded in the bud. The only suggestion foi- 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, 

 of 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. 

 AAliere 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 injur}^ 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 3"oung 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 cotj'ledons 

 increase in size and the hypocotyl becomes much thickened. In some 

 cases 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 planta are 10 inches or a foot 



[Cir. ll'Oj 



