THE AXGIOSPERMAE : LEAVES 



963 



to throw much light upon the evolution of the species concerned, certainly 

 not, at any rate, in regard to their remote ancestry, though they may indicate 

 comparatively recent changes of foliage type in the history of the race. 



The case of the Ivy, mentioned before, may be considered as falling into 

 this class, inasmuch as the palmate leaves, arranged in two ranks, which are 

 produced on the climbing vegetative shoots, are naturally characteristic of 

 the earlier stages of development, while the entire ovate leaves of the flowering 

 shoots are only produced when the flowering stage has been reached. It is 

 preferable, however, to regard the change in this case as correlated with 

 bract development, which is in every case associated with the flowering stage 

 of the plant, rather than as purely a phenomenon of maturity, for if flowers 

 are not produced, no change of foliage occurs. 



Fig. 9 so. — Artocarpus integrijolia. Leaves of various shapes, 

 illustrating habitual heterophylly. 



Juvenile foliage which gives place to mature foliage in the ordinary course 

 of vegetative development is well known in Pinus (see p. 690) and some other 

 conifers and occurs also in many Angiosperms, especially among woody 

 climbers. The genus Potlios (Araceae) provides a very striking example. 

 Starting with sessile, entire leaves in the young plants, there is first a change 

 to petiolate leaves and later to divided leaves, which are only irregularly 

 incised to begin with, but finally become regularly multipinnate. So great 

 is the difl'erence that young specimens in cultivation have been regarded as 

 types of a new genus until their mature foliage appeared, in the same way 

 that juvenile forms of some Conifers are vegetatively propagated and cultivated 

 under the names of Prumnopitys and Retinospora, which are applied only to 



these juvenile states. 



The genus Eucalyptus consists of large, rapidly growmg tree species, 

 which show a very marked difl'erence between the juvenile and the mature 

 foliage (Fig. 951). The later leaves hang vertically ; they are always narrowly 



