56 DIMORPHIC BRANCHES IN TROPICAL CROP PLANTS. 



from this point of view. Intermediate forms of branches are accom- 

 l^anied by abortion of flower buds and other abnormalities. 



The substitution of additional branches of the vegetative form 

 for the fruiting branches is a frequent occurrence in imported tj'pes 

 of cotton. The plants regain their normal fertility when the normal 

 relations of the branches are restored. This readjustment of the 

 habits of branching represents one phase of the process of acclimati- 

 zation. 



The dimorphism of the branches is also a factor in the problem 

 of weevil resistance, since the development of larger or more numer- 

 ous vegetative branches tends to render the crop late. Early crops 

 usually suffer less injury from the weevils. 



In the Central American rubber tree (Castilla) the axillary 

 branches do not share all the functions of the main stalk or trunk of 

 the tree. The axillary branches bear the flowers and fruit, but are 

 shed after a few seasons. The permanent branches always arise 

 from extra-axillary positions and usually do not begin to develop 

 until the tree is several years old. 



The self-pruning habit of the Central American rubber tree is an 

 important cultural advantage. Only an occasional tree requires prun- 

 ing, and then only to correct accidents or abnormalities. 



In the coffee tree only vegetative branches, or uj^rights, like the 

 primary trunk, are produced from the true axillary buds. All the 

 fertile branches, or laterals, have extra-axillar}^ positions above the 

 true axillary branches. Lateral branches can not produce uprights, 

 nor can new laterals be produced from old uprights. 



As the crop is borne only on young wood of lateral branches, a 

 vigorous growth of lateral branches must be maintained if good crops 

 are to be secured. New uprights must be formed to produce new 

 laterals, or laterals may continue to grow and subdivide if the growth 

 of uprights is prevented by pruning. Failure to take the dimorphism 

 of the branches fully into account in the work of pruning often 

 results in serious injury to coffee plantations. The practical value of 

 the different systems of jDruning the coffee tree depends on local con- 

 ditions of climate and soil, as well as upon the quality and cost of the 

 labor supply. 



In the cacao tree fruit twigs may be borne on all parts of the old • 

 wood, including that of the main trunk, but there are two types of 

 vegetative branches. The upright growth of the trunk takes place 

 by a series of shoots, each of which is terminated by a whorl of three 

 to six branches. A new ujiright shoot arises from the side of another 

 upright, not from a whorl branch. 



The natural habit of growth of the cacao tree, by a succession of 

 whorls, is very undesirable in plantations, and can be avoided by 



198 



