Mason — Ligneous Zonation and Die-Back in the Lime, West Indies. 261 



of the sections in areas presenting all the characters of "spring wood," is 

 evidently not especially associated with the absence of the die-back condition, 

 for it is particularly marked in the wood from the coastal region of O'Garra's, 

 where the trees were badly affected. Inasmuch as nothing definite is known 

 concerning the factors which determine the anatomical differences between 

 "spring" and "autumn" wood, even in temperate regions, it would be obviously 

 unprofitable to attempt to discuss this matter further. 



It will be also clear that the diameter of the vessels is subject to coiisiderable 

 variabilitj^, and that, though it is generally greater in healthy trees than in 

 those showing the die-back condition, yet no significant relationship is to be 

 traced. 



It was suggested in the preceding section that the parenchyma bands 

 originated during periods of desiccation. Jeffrey (3) has pointed out that the 

 impulse towards production of terminal parenchyma was probably supplied in 

 the past bj'' climatic cooling. It would seem that the impulse needed for the 

 production of tangential parenchyma may also be provided by desiccation of 

 the cambium. It is not improbable, of course, that the production of parenchj-ma 

 as a result of climatic cooling is in the last analysis effected by desiccation. 



It will be evident that, if the interpretation of the zonation adopted is correct, 

 namely, that the zones of vessels correspond to periods during which the plant's 

 water-supply was adequate, and that the bands of parenchyma were conditioned 

 by desiccation of the cambium, their frequent association must indicate very 

 rapid changes in the aridity of the environment. Rather sudden fluctuations 

 do, in fact, characterize the dry season. There is generally at this time a bare 

 sufficiency of moisture for growth. It may be suggested that the production of 

 tangential parenchyma is due to autogenic changes in the activity of the cambium, 

 or that it is possibly related to the eircumvasal parenchjona, but there would 

 seem to be no foundation for such a view. 



It will be recalled that the parenchyma bands were generally more pro- 

 nounced and more irregialar in their radial distribution in those habitats in 

 which the aridity of the environment fluctuated most markedly. It is in these 

 habitats that the die-back condition is most conspicuous. It would seem that 

 the rapid and repeated desiccation of the meristems, which is recorded in the 

 case of the cambium by tangential parenchyma, results in the premature loss 

 of dominance of the apical over the lateral buds, as a result of which daughter 

 shoots grow out and replace the mother shoot. A repeated dying back of shoots 

 of this nature is characteristic of trees affected with die-back. 



The growth of the Lime in the West Indian islands of- St. Vincent and 

 Carriacou is of some interest in this connexion. The islands lie only some fifty 

 miles apart, but differ very markedly in two respects. The annual rainfall in 

 St. Vincent is about 100 inches, and the soil is shallow and porous, drying out 

 with great rapidit^^ whenever a couple of weeks pass without rain. The annual 

 precipitation in Carriacou, on the other hand, is only about 40 inches, but the 

 soil is very retentive, and dries out slo\^'ly. The consequence is that in the dry 

 season St. Vincent undergoes rapid though normally short-lived spells of 

 drought, whereas in Carriacou drought conditions are initiated relatively slowly 

 as a result of the retentive natiire of its soil, but are frequently prolonged. 

 Now the Lime cannot be grown in St. Vincent, though repeated attempts have 

 been made to do so, while it thrives in Carriacou, where the aridity is many 

 times greater. It would seem that this plant can tolerate considerable aridity 

 and still thrive, but that it is unable to withstand a rapid desiccation of its 

 tissues. The writer is unaware of any other hypothesis that so well accords 



