14 RESULTS OF COTTON EXPERIMENTS IN 1911. 



larly undesirable in wee^dl-infeste(l I'egions, where a prompt develop- 

 ment of the crop is of the utmost importance in avoiding weevil injury. 



Though generally confused with a distortion of the leaves caused 

 by plant lice, the juvenile leaf-curl is quite independent of the presence 

 of these insects. Both lands of malformation are often to be found 

 on the same plant, but they are not of the same character. The juve- 

 nile leaf -curl may be <listinguished by the fact that it is usually accom- 

 panied by irregular holes or incisions in the leaves that do not occur 

 when plant lice alone are involved. Nothing has been found to indi- 

 cate that the juvenile leaf -curl is due either to insects or to parasitic 

 fungi or bacteria. All the damage is done, apparently, while the 

 leaves are still in very early stages of development. The parts that 

 remain uninjured appear to be entirely normal. 



The underlying cause of the juvenile leaf -curl is still obscure, but it 

 is evident that the malformations result from the death of some of the 

 cells of the leaves during the early stages of grow^th. If only a few 

 cells die, the result is a simple puncture of the leaf surface. The loss 

 of larger groups of cells results in more serious malformations and 

 even in the abortion of the whole terminal bud. Thus the distorted 

 leaves and the aborted terminal buds may be considered as merely 

 different symptoms of the same form of injury to cells of the seedlings. 

 The petioles of the leaves show another injury in the form of long 

 narrow gashes, as though cut lengthwise with a knife. It is to be 

 expected that corresponding malformations will be found in young 

 plants of other species. Mr. G. N. Collins has called the writer's 

 attention to a peculiar lobing of the leaves of Indian corn, which may 

 be of the same nature. 



That the injuries begin at very early stages in the development of 

 the leaves is shown by the fact that wounds are often healed, or some 

 of the lost parts may be regenerated from adjacent tissues. As a 

 result of healing the wounds, the leaves often have a peculiar broken- 

 and-mended appearance. A similar condition is sometimes found 

 in the leaves of abnormal hybrids and mutations, but in these cases 

 the abnormality continues through the whole life of the plants. This 

 analogy might be taken as a confirmation of the idea that the cause 

 of the malformation is to be found in the injury or death of some of 

 the cells at very early stages of the development in the bud, while it is 

 still possible for new cells to be formed by proliferation. 



The popular belief that the leaf-curl is much more severe during 

 periods of cold weather may mean that some of the cells are killed by 

 cold. Or it may be that cold has the effect of delajmig division or 

 other cytologic al processes that normally occur at night. Such a 

 delay would render the cells more susceptible to injury from sudden 

 exposure to heat or sunlight in the morning. It has been noticed 



[Cir. 96] 



