LEAF-CUT, A DISOEDER OF COTTON SEEDLINGS. 31 



lie in a somewhat radiating; position between the principal veins. 

 Healing of wounds and regeneration of lost parts show that the 

 injuries are liable to occur at a very early stage in the development 

 of the leaf. Sometimes an extensive new growth or regeneration 

 takes place, resulting in a curious doubling or overlapping of lobes 

 of injured leaA'es. The power of the injured tissues to heal is also 

 responsible for adhesions between parts that lie folded together in 

 the bud. These adhesions are usually responsible for failure of 

 normal expansion of the blade. None of these secondary symptoms 

 occur Avith the leaf-curl induced by plant lice. 



CAUSES OF LEAF-CUT INJURIES. 



Leaf-cut is hardly to be reckoned as a disease unless the word is 

 used in its most general application that includes any departure from 

 normal structure or function. Neither of the two general classes 

 into which diseases are usually divided, constitutional and parasitic, 

 would include the leaf-cut. Though some of the cells are destroyed, 

 the remaining tissues of the plant do not become abnormal in any 

 way, and there is no indication that parasitic organisms of any 

 kind — bacteria, fungi, insects, mites, or wonns — are involved. Another 

 class of ecological disorders may need to be recognized, intemiediate 

 between physiological diseases and mechanical injuries or trauma- 

 tisms. Leaf -cut is a disease only in the sense that frostbite, snow 

 blindness, and other environmental injuries are to be considered as 

 diseases. 



Young cotton plants are often subjected to extreme conditions 

 during the early stages of gi'owth, when the leaf-cut injuries occur. 

 The leaves and roots are still close to the surface soil, where they 

 can be chilled at night and scorched in the daytime. Cold nights 

 are sometimes looked upon as the cause of the injury, and may be 

 an intensifying factor, but the sudden heat of a bright morning sun 

 seems more likely to kill the cells of the young leaves than low tem- 

 peratures during the night. Leaf-cut often affects late plantings 

 long after the night temperatures have ceased to approach the freez- 

 ing point. It has been noticed that exposure to a bright morning 

 sun after a cold night will tlirow cotton seedlings temporarily into 

 a wilted state, doubtless because the leaves lose Avater by transpira- 

 tion faster than it can be absorbed by the chilled roots. Leaf-cut 

 seems to be especially prevalent under such conditions. 



That leaf-cut is in some way connected with exposure or wilting 

 of the delicate tissues is also shown by the fact that tlie injuries are 

 most severe and occur most frequently along radiating lines midway 

 between the principal veins. These lines of greater susceptibility 

 represent the most exposed parts of the upper surface of the young 



[Cir. 120] 



