328 



Glaucousness. 



The only reference to Eucalyptus I can find in the " Life and Letters of 

 Charles Darwin" is in the following passage, iii, 341, in a letter to Asa Gray (the 

 celebrated American botanist) : — 



. . . I am now trying to make out the use or function of "' bloom " or the waxy secretion on 

 the leaves and fruit of plants . . . Are such plants commoner in warm than in cold climates? I ask 

 because I often walk out in heavy rain, and the leaves of very few wild dicotyledons can be here seen with 

 drops of water rolling off them like quicksilver. Whereas, in my flower garden, greenhouses and hothouses 

 there are several. Again, are bloom-jtrotected plants common in your dry western plains? Hooker 

 thinks that they are common at the Cape of Good Hope. It is a puzzle to me if they are common under 

 very dry climates, and I find bloom very common on the Acacias and Eucalypti of Australia. Some of the 

 Eucalypti which do not appear to be covered with bloom, have the epidermis protected by a layer of sonic 

 substance which is dissolved in boiling alcohol. Are there any bloom-protected leaves or fruit in the 

 Arctic regions? 



Darwin's experiments and observations on bloom remained unfinished at the 

 time of his death. 



The colour of the leaves in Eucalypts is subject to many variations. The fundamental colour is 

 without doubt green, but it is often concealed by the secretion of essential oils in the glands, not very 

 visible, with which the foliage and all the young parts of the tree are enveloped. The result is glaucous 

 tints of different degrees, which sometimes give a bluish cast to the trees, whence the name Blue Gums 

 given by the Australian colonists to some srjecies, among others to E. globulus, which, in its juvenile stage 

 shows this character very markedly. Sometimes the oily resinous secretion exudes and becomes solidified 

 in the form of a whitish jmlverulence, and there are indeed some Eucalypts having this last colour in a 

 very pronounced manner. These are the White Gums of the Australians. (In a preponderating number 

 of cases, when an Australian speaks of a White Gum, he means one with a white, smooth lark, J.H.M.). 

 Other variations of leaves should be noticed. If these organs appear glaucous, glaucescent, greyish, or. 

 almost white in certain species, it is because of an oily resinous secretion which does not sensibly alter the 

 fundamental green colour. These have green foliage, often shiny, and even varnished, particularly on the 

 upper surface ; in the case of many of the others, the colour remains dull, and it is almost the same colour 

 on both sides of the leaf, especially when the blade, being in an oblique or vertical plane, receives almost 

 as much light from one side as the other. In the species in which the blade is horizontal, the lower side is 

 always duller than that which faces the sun. (Naudin, 2nd Mem., 10.) 



In certain species of Eucalyptus (E. globulus and E. pulverulenia) the outer wall of the epidermis 

 is provided with a coating of wax; the latter consists of an aggregate of small rod-like particles. (Soledercr, 

 i, 352.) 



Many leaves, especially those that appear " glaucous " have a bluish-grey surface film of wax, 

 sometimes known as bloom, which is readily removed by rubbing. Wax coats are best developed by 

 xerophytes, and appear to be increased by excessive transpiration. Thin as they are, wax coats effectually 

 impede transpiration, the mere rubbing of a glaucous leaf sometimes inducing an increase of a third in the 

 transpired water. Wax coats also retard the heating of leaves. As with hairs, but not with cutin, wax 

 coats are best developed on the under leaf surface, where the stomata are the more abundant. (Coulter, 

 Barnes and Cowles, ii, 570.) 



One of the most interesting Eucalypts in this (the author is speaking about Horizontal and Vertical 

 Leaves) connection is E. pulverulenta (E. pidvigera) which is growing in the Mountain Region at Cox's River, 

 at Bathurst and near Cooma. This tree appears to have been unable to develop any lanceolate leaves 

 at all, or to substitute the alternate for the juvenile opposite arrangement, the whole of its foliage being 

 either orbicular or broadly ovate, and being sessile, the cordate leaves remain clasping the stem at right 

 angles, and therefore present their full surface to the sun. It is now that we see the potentialities of the 

 Eucalypt to adapt itself to its surroundings, and the method selected in this instance has been to cover 

 the leaves with a glaucous powder or vegetable wax which reduces the effect of the sun's rays and therefore 

 lessens the evaporation, while it also serves to keep out the cold in winter. It would seem, however, that 



