48 



THE COCO-NUT 



CHAP. 



But there is no good evidence that the region in grave peril is 

 widening downward. 



Further evidence that humidity is a condition of contagion 

 is found in the fact that young trees are more susceptible than 

 old ones. About the crown of old tall trees the air moves 

 comparatively freely, and keeps them comparatively dry. For 

 every old tree killed, two which are just coming into fruit, or 

 would do so within a year or so, die, although the total number 

 of such trees is naturally much less than that of adults. On 

 the other hand, very young trees, of two years or less, are com- 

 paratively rarely attacked. Since humidity is the condition of 

 easy infection, the extension of the disease occurs principally 

 during the most humid season. 



The first symptom, always in young trees, and almost always 

 in old ones, is the yellowing and wilting of the youngest still 

 folded leaf. The disease attacks the soft undifferentiated tissue 

 of growing points. It is likely that infection normally occurs 

 where the germs can get direct access to these points without 

 penetrating through mature tissue ; but the germs might also 

 be borne by insects which could carry them in through 

 mechanical lesions in old tissue. In young trees the youngest 

 leaf presents the only possible path of direct unaided infection ; 

 and, however infection may occur, the youngest leaf is directly 

 inserted in the tissue susceptible to rot, and must die before 

 the rot embraces the most of the soft tissue commonly called 

 the cabbage. As soon as the youngest leaf is noticeably dis- 

 coloured, it can easily be drawn out. The next youngest leaves 

 follow in rapid succession. Within from two to four months 

 after the disease can first be detected, the most of the leaves 

 will have fallen. A few of the oldest leaves grow from tissue so 

 hard that the rot makes little or no progress in it ; these 

 leaves, four to a dozen in number, persist for months after the 

 younger leaves are gooe. It is in this stage, with a thin whorl 

 of old leaves crowning the stem, that the most of the diseased 

 trees are found. These leaves very likely fall only when their 

 natural time comes, uninfluenced by the rot. 



In the case of old trees, the young flowering branches, like 

 the youngest leaves, spring from the soft heart. Infection can 

 occur along these branches, and they can give the first external 

 sign of disease ; but these are exceptional cases. Branches 

 whose nuts are more than half grown are grounded in tissue 

 so mature that the disease does not usually prevent the nuts 

 from ripening. But no new nuts are set after the appearance 

 of the rot, and the youngest nuts almost or quite always fall 

 without becoming ripe. 



