44 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



These facts may explain why some of the more definite but less 

 striking differences have been overlooked, notwithstanding the 

 attention that has been given to the study of the more reduced and 

 apparently more specialized forms of leaves. 



In addition to bud scales and prophylla, special names have been 

 given to the reduced leaves of underground shoots (kataphylls) and 

 to those that subtend flowers or inflorescences (hypsophylls), but 

 these terms seldom, if ever, refer to examples of definite dimorphic 

 differences like those that sometimes exist among true fohage leaves, 

 nor do they serve to distinguish gradual changes of characters from 

 those that are more definite and abrupt. 



It is convenient to use a general term (hypophyll) to cover all 

 forms of reduced leaves, since nearly all plants have such leaves, in 

 addition to the true foliage leaves (trophophylls) and the floral 

 leaves (anthophylls). ]\Iost hypophylls are formed by the reduction 

 or suppression of the blade and petiole of the leaf, while the sheath 

 or the stipules are retained or enlarged, as in the involucral bracts of 

 the cotton plant. Both the hypophylls and anthophylls may be 

 disregarded in the study of differences among the true foliage leaves. 

 The terms that have reference to various kinds of differences 

 among the leaves and branches of the same individual plant may 

 be summarized briefly as follows: 



IIeteroph)jlhj, a general term covering all kinds of diversity of leaf 

 forms on the same plant without regard to whether the difl'erences are : 

 definite or adaptive. \^; 



Clinomorphy, a general term for differences of form arising through :; 

 oblique or horizontal position. ^ 



Laterality, a general name for special characters of lateral branches 

 as distinguished from those of an upright trunk or branches. '*"" 



Anisophylly, inequality of leaves on ui)per and lower sides of hori- 

 zontal or oblique shoots, as in Broussonetia. (See fig. 18.) 



Ilypophylly, the production of rudimentary or reduced leaves, 

 including })roj)hylls, bud scales, bracts, and other less common condi- 

 tions, such as the scale leaves above the cotyledons of seedlings of 

 Persea gratissima and Citrus trifoliata and those near the ends of 

 upright shoots of Theohroma cacao. 



PhyUoropy, production of two or more definitely different kinds 

 of foliage leaves on the same plant, as in the cacao tree and in narrow- 

 leaved varieties of cotton, okra, and Hibiscus cannahinus. 



Cladoropy, production of two or more definitely difi"erent kinds of 

 branches on tlie same i)lant, as in cotton, coffee, cacao, and Castilla. 

 Cladoptosis, the self-pruning habit or spontaneous falling oft" of spe- 

 cialized temporary branches, as in Populus, Quercus, and Castilla. • 



221 



