DISEASES OF LIME AND OTHER CITRUS TREES 197 



by the weight of foliage and fruit the secondary branches in turn 

 lose their upright position and are replaced by further vertical 

 shoots from themselves or from the original branch further back. 

 There is thus a continual tendency to the opening of the centre 

 of the tree and its filling with new vertical branches. Where 

 there is room for full development the mature tree is typically 

 dome-shaped, with its outer branch system supported by a num- 

 ber of more or less equal stems of the first or second order, 

 springing usually from points not far above the crown. As the 

 outer ends of the lower branches become bent into or below the 

 horizontal position the outward growth necessary to keep the 

 foliage in rank with that of the branches above it (it would 

 otherwise be overshadowed) is continued in the same way by 

 shoots arising from points situated on the curve of the branch. 

 This succession of the terminal shoot by a shoot produced further 

 back is in fact the principal characteristic of the growth of the 

 tree. 



Its importance from the point of view of duration lies in the 

 fact that, whereas in a vigorous tree the later shoot supplements 

 the leader, in one which is lacking in vigour it tends to replace 

 it. The younger shoot seems to establish a pre-emption on the 

 ascending current of sap, and, when the supply does not suffice 

 for both, the terminal part of the older shoot beyond the junction 

 is stopped in its growth and during some spell of dry weather 

 drops its leaves and dies. By repetition of this process it is 

 not uncommon for the axis of a long basal branch to be made up 

 of successive sections representing ten or more orders of branching 

 and to come at last to bear an insignificant tuft of foliage. 



For a time the growth about balances the losses and then 

 begins to fall behind. There is considerable recovery during 

 the wet season, which may be maintained in favourable years 

 or more than lost when severe droughts occur. On the whole 

 the decline, once it has set in, continues, and as the older branches 

 are reached there is a marked acceleration of the rate of loss. 

 The bearing of a heavy crop of fruit, especially in dry weather, 

 is frequently followed by a sudden loss of branches, and this 

 effect may to some extent account for the rapid failure which 

 has often been observed to follow upon heroic attempts to restore 

 the trees by cultivation and heavy manuring. 



The dieback is closely accompanied by Diplodia infestation, 

 and when a branch of any considerable size fails, unless it is 

 promptly removed flush with the older branch from which it 

 springs, the sector of wood directly connected with it, and the 

 overlying bark, dry out and are found infested with the fungus, 

 so that long lesions the width of the dead branch above and 

 running out to a point below are produced on the older member 

 and are usually irreparable. Associated with their development 

 there is commonly a considerable flow of gum. 



The inability of the fungus to attack wood and bark which 



