ABRUPT CHANGES OP LEAF FORMS. 



13 



the leaves were badly luiitilated by the bites of insects. It was also 

 necessary to search a littk^ farther to find examples of more gradual 

 transition from the entire to tlie divided state. (See PI. II.) 



Those who })refer matliematical statements of such facts might 

 measure the deptlis of the incisions of the leaves and construct 

 curves or other numerical expressions of the differences of form, 

 but the nature of the differences is apparent in the photo(:;raj)hic 

 reproductions. It is evident from the abrupt 

 ness of the transition that curves representing 

 measurements of the divisions of the leaves 

 would show two very distinct and well- 

 separated modes, quite as distinct as those that 

 would represent the expression of contrasted 

 characters in cases of ISfendelian segregation in 

 the second generation of a h3"brid. 



It is difficult to imagine that any practical 

 advantage can be secured by the plants by 

 changing the form of the leaves thus abruptly 

 part way up the stalk. Yet it is possible that 

 the different forms of the leaves mav be con- 

 nected wdth the fact that there is a difference 

 of function among the internodes of the stalk. 

 The internodes of the upper part of the stalk 

 produce fruit or fruiting branches, while those 

 of the lower part do not. Some of the lower 

 internodes of the cotton stalk give rise to large 

 vegetative limbs with the same functions as 

 the stalk, while other internodes produce only 

 small abortive branches or none at all. Sev- 

 eral of the barren mternodes usually intervene 

 between the highest of the vegetative limbs and 

 the lowest of the functional fruiting branches, 

 as though it were difficult to change abruptly 

 from one form of branches to the other. 



In Deccan hemp and the okra plant the 

 fruits are borne directly at the axils of the 

 main stalk without the intervention of fruiting 

 branches. It may be that the divided leaves indicate in advance the 

 internodes that are to produce flowers and fruit. Change of leaf 

 form marks the approach of the fruiting condition in such plants as 

 Hedera helix and Ficus repens, but in such cases the change of leaf 

 forms does not occur on the same axis of growth. The creeping 

 stems of the juvenile stage represent an adaptive condition inter- 



''21 



Fig. 2.— Four leaves from suc- 

 cessive internodes of the same 

 stalk of Ilihixcus cannabinus, 

 showing slight dilTerences 

 among the simple leaves and 

 abrupt change to the divided 

 form. (Natural size.) 



