412 



Resin. 



Coats of varnish us protective coverings are especially to be met with on young 

 leaves, which they guard from over-transpiration and desiccation during their 

 development; and when the leaf-laminae become provided with a cutieularised 

 epidermis, these coats disappear. 



" The loaves of many desert xerophytes are coated with resin, and often have a varnished aspect, 

 shining in the sunlight. The factors influencing the formation of resin-coats are unknown. Like wax- 

 coats, they may retard transpiration, and it has been suggested that they reflect light, an excess of which 

 may injure the chlorophyll." (Coulter. Barnes and Cowles' " Text Book of Botany " II, 571). 



I do not know whether the chemist has actually isolated a resin from Eucalyptus 

 leaves ; apparently it must be rare in the genus, and the Tasmanian E. vernicosa Hook f. 

 ■arould be a possible source. Mitchell (quoted by Bentham, B.FL, hi, 225), in " Three 

 Expeditions/ 1 ii, 175. speaking of E. alpina Lindl., which he discovered, says "with 

 short, broad, viscid leaves." They are both from sub-alpine localities. 



Shape. 



Authors have not much to say, in a general way, concerning the shape of mature 

 Eucalyptus leaves : — 



"'. . .in the adult shrub or tree of most species vertical (or sometimes horizontal), alternate, 

 petiolate, and passing more or less from broadly ovate to lanceolate acuminate and falcate, always rigid 

 whether thick or thin, penniveined, the midrib conspicuous . . ." (B.FL iii, p, 185). 



" The form, size and venation of the leaves described have always been taken from those of the 

 flowering branches of what have been supposed to be adult trees or shrubs; when not stated to the 

 contrary, they are always alternate and petiolate." (lb. p. 186). 



The following passage refers to leaves in general. 



Variation in Shape. " Foliage leaves . . . exhibit an almost inexhaustible variety in their 

 internal structure and external form, a fact partly due, no doubt, to the multifarious duties they have to 

 discharge. The most important of all these functions is the manufacture of organic materials from 

 inorganic food . . . 



That those members of the plant to which is allotted the manufacture of organic matter should 

 exhibit such a marvellous diversity can hardly astonish us: for how infinitely varied are the conditions 

 under which this function is performed in the different zones and regions of the globe. Even within the 

 narrow confines of a restricted area we may t\nd habitats damp and dry, sunlit and shady, tranquil and 

 tempest-tossed. Nor should we be surprised to find leaves of diverse shape at different heights on one and 

 the 3ame shoot, and t ba1 the foliage borne by any plant may exhibit variations in form in succcssi ?e seasons 

 of the year." And so on. (Kerner and Oliver, I, 626). 



In other words, in shape of leaves, the ideal is not uniformity, or " comparative " 

 uniformity. 



The vast majority of mature Eucalyptus leaves arc falcate-lanceolate in shape, 

 and those of different shape may be looked upon as exceptions. Sometimes the 

 evolution towards the narrower form is retarded, and hence I have, on occasion, used 

 the expression " Retarded Heteroblasticity." 



