962 A TEXTBOOK OF THEORETICAL BOTANY 



water or food storage, climbing or the capture of insects. Some adaptive 

 modifications of this type we shall deal with later in this chapter. 



The second type or habitual heterophylly involves the formation on 

 the main shoots, of foliage leaves of different sizes, and to a lesser extent, of 

 different shapes, apparently without any special functional significance. 

 Among the commonest cases of such pure heterophylly are those of plants 

 with opposite leaves in which one leaf of a pair is smaller than the others. 

 This is quite common on plagiotropic* shoots, where it is normally the upper 

 leaves which are reduced in size. This is most pronounced in those species 

 where the leaves on plagiotropic shoots are twisted laterally, so as to form 

 flattened shoots on which there are two lateral rows of large leaves, and two 

 dorsal rows of small leaves, representing unequally developed leaf-pairs. 

 This lateral heterophylly may perhaps confer an advantage by avoiding the 

 overshadowing of leaves by other leaves but it is not inseparable from this 

 condition, because habitual heterophylly is also found on the orthotropic 

 shoots of many plants where the factor of overshadowing does not come in. 

 The reduction of one member of each pair of leaves often goes so far as its 

 complete disappearance, the originally opposite leaf arrangement being 

 sometimes traceable by the presence of supernumerary stipules at each node 

 and sometimes, as in the Elm, traceable only during the development of the 

 young plant, in which a primitively isophyllous, opposite arrangement of 

 the leaves gives place, at an early stage, to an alternate arrangement with 

 leaves in two ranks only, through a short intermediate zone of true hetero- 

 phylly. These latter cases are of particular interest in that they suggest 

 the possibility that alternate leaf arrangements in which no trace of the change 

 now remains, may have been derived from the opposite arrangement. 



A very marked, but exceptional, type of habitual heterophylly is exhibited 

 by certain plants which bear leaves of variable form, distributed at random. 

 Three well known examples of this are : Broussonetia papyrifera, (the Paper 

 Mulberry), Avtocarpiis integrifu/ia (the Jack Fruit) and Louicera japonica, the 

 last named producing irregularities especially under starved conditions. In 

 all these cases the normal leaf shape is entire, but only a minority of the 

 leaves retain this form, the majority developing in most irregular fashion, 

 various parts of the lamina being suppressed, so that hardly any two leaves 

 are alike (Fig. 950). 



Before leaving this type of heterophylly we may also mention cases where, 

 as in the common Ivy, the leaves of flowering shoots, although retaining the 

 full size and character of foliage leaves, differ in outline from those of 

 vegetative shoots. Such differences probably merge into the widespread 

 condition of the production of specialized bracts associated with the flowers. 



The third type or developmental heterophylly is that associated with 

 the change from juvenile to mature foliage in the development of the individual. 

 This touches upon the vexed question of evolutionary recapitulation during 

 ontogeny, but with a few exceptions the changes involved are not such as 



* The term plagiotropic refers to shoots of horizontal growth. The opposite term, 

 orthotropic, refers to shoots of vertical growth. 





