256 



Scientific Proceedings, Royal Dublin Society. 



healthy state. It is probably unnecessary to point out that nearly every stage 

 between trees in perfect health and those showing the most pronounced dying 

 back of the branches is to be encountered. The collection of shoots was made 

 with a view to an examination of the differentiation of the xylem cylinder. 

 It was argued that if i^ronounced fluctuations in the water balance of the 

 plant (6) were the cause of the affection, these fluctuations might be registered 

 hj different types of zonation in the woodj' cylinder. 



Periodicity of Groicth. 



Before describing the results of this examination it seems desirable to say 



something concerning the periodicity of growth and its relation to moisture 



. conditions. Tempei'ature fluctuates so- little in these islands in the course of 



the year that it does not obtrude itself as a factor. Reference to fig. 1 will 



Fig. 1. 



show that there are two periods in the course of the year at which the rate of 

 fruit production becomes markedly accelerated. In other words, there are two 

 crops, both of which ai'c gathered in the wet season. Inasmuch, however, as 

 the fruit requires some five months to mature, the earlier portion of the first 

 crop may develop under rather arid conditions. The large values for the 

 coefficients of variability in rainfall from February till June, when considered 

 in relation to the meagre rainfall at this time, will serve to indicate how 

 frequently spells of drought occur. 



Vegetative growth (leaf production and shoot elongation) commences some 

 time after the maturation of the fruit, and is generally most active between 

 January and August — that is to say, in the dry season. It will thus be seen 

 that the period of maximum vegetative activity is also the time at which spells 

 of drought are especially liable to occur, 



