4'2 DIMORPHIC BRANCHES IX TROPICAL CROP PLANTS. 



them to send out secondary or lateral branches near the base and thus 

 assist in forming a compact, wdl-shaped crown. Witli t\v(i (ir more 

 strong branches from near the base of a single whorl branch a condi- 

 tion somewhat similar to the original whorl may develop, but essen- 

 tially difl'erent in the subsequent habits of growth, since these 

 branches do not tend to spread apart like true whorl branches, and 

 are able to continue the upward growth of the tree without the forma- 

 tion of any more upright shoots from the main trunk. 



A further indication that the habit of forming the whorled 

 branches represents a definite specialization may be found in the 

 fact that the upper leaves of an upright are often aborted. The 

 stipules are of the normal size, but the petioles and blades do not 

 develop. The stipules soon drop oft', leaving small scars on the sur- 

 face of the bark as the only indication of the joints. 



It is not clear whether this habit of forming abortive leaves is to be 

 viewed as an adaptation to avoid the clustering of too many leaves 

 at the top of an upright shoot, or is connected with the shortening 

 of the internodes to form the whorl of branches. AMien the leaves 

 are aborted many short internodes are likely to be formed below the 

 whorl. In other cases there are no abortive leaves. Even the whorled 

 branches may arise from axils of normal, full-sized leaves, but in 

 such cases the whorl is likely to be somewhat irregular, as though the 

 internodes had not been sufiiciently shortened. 



If these reduced leaves are taken into account, the cacao tree may 

 ^be said to have three kinds of leaves, the leaves Avith the long petioles 

 on the lower parts of the uprights, aborted leaves at the ends of the 

 uprights, and short-pet ioled leaves on the whorled branches. The 

 specialization of the leaves of the cacao is somewhat similar to that 

 of the pine tree. Young seedlings and new shoots of pines that have 

 been cut down or severely pruned have functional green leaves all 

 along the shoot. Ordinary shoots and branches of pine trees have 

 no functional green lea^ es. but only scalelike membranous sheathing 

 leaf bases. The functional leaves of adult pine trees represent the 

 terminal clusters of a few leaves at the ends of very short specialized 

 branches that appear to be incapable of further growth. New 

 branches have to be developed from special zones where the axillary 

 buds of the leaves of the uprights remain dormant instead of pro- 

 ducing the short leaf-bearing branches. 



The habit of the cacao tree to produce the long uprights with a 

 whorl of branches at the end appears thoroughly imdesirable from 

 the cultural standpoint, but if we consider the habit of the wild cacao 

 to grow in dense thickets with many other kinds of woodv vegetation 

 its peculiar habit of growth may be seen to have some advantages. 

 The rapid growth of the upright shoots enables a cacao tree to raise 



198 



