156 



THK INSTABILITY OF LEAF-MORPHOLOGY, 



Transpiration. — -The preventive measures adopted by plants 

 against excessive transpiration are responsible for similarity, or 

 variability in leaf-characters, according to the resultant effects 

 of uniformity or opposition, arising from the methods used to 

 regulate the loss of moisture. 



Adaptahility. — The plant possessing the ability to exist in a 

 varied habitat must, of necessity, be capable of modifying its 

 leaf-structure to conform with the changes in the environmental 

 conditions, the phrase "common and variable" being exception- 

 ally applicable to adaptable species. 



Teratological. — The meristematic attack of microscopic insects 

 probably plays a more important part in the moulding of leaf- 

 characters than has been generally recognised. Leaf-twisting, 

 and axial disarrangement of the alternation of whorls, may be 

 set up by insect-attack or mechanical injury. 



Most of the above factors are capable of demonstration, but 

 there remain, the uncertain effect of heredity, and the still less 

 known equation "Mutation," to exercise a disturbing influence 

 on leaf-characterisation. The changes brought about by natural 

 hybridisation, and cross-fertilisation, though still largely a matter 

 for conjecture, cannot be disregarded in a consideration of the 

 morphology of leaves. 



Examples. 



RANUNCULACEiE. 



The marginal toothing, the leaf-character separating Clematis 

 aristata R.Br., from C. glycinoides DC, is inconstant; and vari- 

 ation in the leaflets of the latter has already been noted. The 

 foliage of Ranunculus pi ebeius R.Br., and that of E. hii't^is Bks. 

 and Sol., are difficult to separate, hairiness, the character chiefly 

 relied upon, being subject to age, and environmental conditions; 

 the dissection of the leaves varies on the individual plant. 



DiLLENIACEiE. 



The xerophytic leaves of several Hibbertias are so similar as 

 to be of little value to the systematist; those of H. linearis R.Br., 

 and H. obtusifolia DC, approach each other, and are finally 

 inseparable. 



