72 DISEASES OF CROP PLANTS 



A marked example of the action of the physical environment 

 on the life and vigour of a perennial plant is afforded by the life 

 history of lime trees in t'ae drier islands, described in another 

 section. When they sur\ive the initial critical period of two or 

 three years the trees become established and flourish for about 

 ten years, after which their vigour declines and they gradually 

 die back, the process being accelerated by, but not dependent 

 on, the scale insect and fungus infestations to which their les- 

 sened resistance renders them liable. No agricultural measures 

 have availed to arrest this decline, which by comparison of 

 locahties is judged to be the effect of irregularity of conditions, 

 especially of the reduction of atmospheric humidity in the dry 

 season. 



Another notable case relates to orange trees in Dominica, 

 which on their own roots, but not when grafted on sour orange 

 stocks, are invariably limited to a relatively short period of healthy 

 maturity by the oncoming of gummosis of the collar. 



Conditions simulating the effects of disease may be produced 

 by genetic changes, perhaps to be explained as loss mutations, 

 taking place even in what are believed to be pure lines. Ex- 

 amples of this are the " man cotton " and " crinkled-dwarf " 

 rogues in Sea Island cotton described by Harland. In the latter 

 case the plant is severely dwarfed, bears yellowish-green, crinkled 

 and ragged leaves much smaller than the normal, and is rendered 

 nearly sterile by shedding. The " man cotton " differs from 

 normal in the smallness of the vegetative parts, with complete 

 male and nearly complete female sterility. Typical]}'' it is a tall 

 lanky plant with very short branches. Both types of rogues, 

 when crossed with normal Sea Island, give a normal first genera- 

 tion. The crinkled dwarf type is known to exhibit simple 

 Mendelian segregation in subsequent generations. 



An example of what may perhaps be called genetic incompati- 

 bility, producing similar results, is afforded by crosses between 

 Sea Island and some local perennial cottons. Part or the whole 

 of the first generation consists of plants with mottled leaves, 

 partly suppressed internodes, and premature cork formation 

 on stems and petioles. The writer's attention has on several 

 occasions been directed to this condition as a new disease. 



