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No. 31. 



LIGNEOUS ZONATION AND DIE-BACK IN THE LIME {CITRVS 

 MEDICA, VAR. ACIPA) IN THE WEST INDIES. 



By T. G. MASON, M.A., Sc.D., 

 Botanist, West Indian Agricultural College. 



(Plates XIII-XVI.) 

 (Bead May 29. Printed August 28, 1923.) 



Introduction. 



Though the Lime thrives on the whole in the more hnmid of the Lesser Antilles, 

 yet its cultivation in those islands which are subjected to periods of pronounced 

 desiccation is fraught with considerable hazard. A progressive dying back of 

 the terminal shoots is veiy prone to occur in habitats in which the aridity of the 

 plant's environment is liable to fluctuate markedly. 



Nowell's (8) view is that the young trees become established and do well 

 for about ten years, attaining to a good size and bearing normal crops. Then, 

 in a uniform, field of this nature, the trees most exposed to the desiccating 

 blast of the Trade Winds begin to show signs of die-back, which sooner or later 

 extends widely, and in two or three years may involve the whole field, or may 

 leave for a time groups of less affected trees here and there. 



The loss of dominance of the apical Ijuds and the replacement of the 

 mother shoots by laterals become especially accentuated. The terminal part of 

 the mother shoot beyond the daughter shoot ceases to grow, sheds its leaves, and 

 dies. For a time the losses incurred in this way may be balanced by the 

 production of new shoots. Partial recovery may even occur, provided the water 

 balance of the plant (6) becomes stabilized, bat normally the decline, when once 

 initiated, continues. The tree dies branch by branch ; a process of self -pruning, 

 which is accelerated, as a rule, by the presence of scale insects and the infestation 

 of the dying shoots by Diplodia, etc. 



Etiology. 



Nowell (8) states that the underlying cause of the decline appears to be 

 one of insufficiency and irregularity of the water supply, and that the duration 

 and completeness of the dry season seem to have more effect that the iirtensity 

 of the wet season. He also points out that a heavy crop of fruit, especially in dry 

 weather, frequently leads to a loss of branches. Spasmodic applications of 

 manure or of cultivation are generally attended by similar results. 



Plan of Work. 



As the writer recently (April, 1921) had occasion to visit a number of Lime 

 estates in the islands of Montserrat and Dominica, a collection of shoots was 

 made from trees showing the die-back condition and from those in a more 



SCIENT, FROC. K.D.S., VOL. XVITj NO, 31, 3 B 



