y66 A TEXTBOOK OF THEORETICAL BOTANY 



certainly the primitive type for the genus, which has been replaced by the 

 specialized phyllode formation. The same replacement of laminar leaves by 

 phyllodes can also be seen in some species of Oxalis, where the primary 

 leaves are ternate. 



Juvenility of foliage is not entirely confined to seedlings, for in many 

 trees similar changes of leaf type may be seen on the vigorous stool shoots 

 that spring from the base of felled individuals. Arising as they do in most 

 cases from dormant buds formed when the tree was young, they frequently 

 display juvenile characters and recapitulate changes observable in the seedling 

 development. 



Along with heterophylly there may be mentioned the asymmetry of leaves 

 which prevails in certain plants, in which the two halves of each leaf, or the 

 two sides of a pinnately compound leaf, are unequally developed. This 

 may occur, as in the Elm, even on orthotropic shoots, but it is much more 

 frequent on plagiotropic shoots. In the latter case it is often associated with 

 dorsiventrality of the plagiotropic axis itself. The genus Begonia provides 

 examples of this in nearly every species, but the relationship to the axis 

 varies, sometimes the larger and sometimes the smaller half of the leaf being 

 upwards. In the latter case, how ever, the larger half is usually turned upwards 

 by secondary twisting of the leaf-stalks. Experiment shows that asymmetry 

 can easily be induced in leaves by severing a vein on one side of the young 

 leaf and thus reducing its nutrition, and in the naturally occurring examples 

 an analogous difference of nutrition seems to be operative, the larger leaf-half 

 being formed towards the better developed side of the dorsiventral axis, 

 from which its trace bundles come. When stipules are present in such cases 

 they are also often asymmetrical, the larger of the two being on the larger 

 side of the leaf. 



Asymmetry has been interpreted as an adaptation to minimise the over- 

 shadowing of each other by closely set leaves. It may have such a value in 

 certain cases, but in orthotropic shoots with widely spaced leaves this need 

 does not arise, nor can it in any case, be regarded as the cause of the asymmetry. 

 Nevertheless the avoidance of overshadowing has a definite biological value, 

 especially with plants growing in poor light. Many plants, especially those 

 with plagiotropic shoots (Fig. 954), whether creeping, climbing, or woody, 

 do in fact place their leaves, by the twisting of the petioles and by dift'erential 

 growth of the petioles in length, in the positions of minimum overlap. This 

 fitting of the leaves together has been given the name of the leaf mosaic 

 and some ecological importance has been attributed to it (Fig. 955). Apart 

 from plagiotropic shoots, the basal rosette of leaves in biennials, (Fig. 956) 

 the so-called " radical leaves," often show very well-marked mosaics, the 

 petioles of the lower leaves in the rosette being so much elongated that their 

 laminae are carried outwards beyond the laminae of the upper leaves. The 

 leaves produced on the elongated flowering shoots produced later by the same 

 plants may not, however, show any mosaic arrangement, unless they happen 

 to be unusually large or closely placed on the stem. 



In treating of adjustments of leaf position mention should be made of 



