434 



NATURE 



{March 12, 1S85 



■xcitation of one brain centre may possibly act in the same way 

 i inhibitory impulse by partially paralysing an adjacent 

 centre. 



Th? For.ns of Leaves 



There are several points in Sir John Lubbock's lecture 

 (Nature, February 26, p. 398) which seem to invite some little 

 criticism. That " the size of the leaf . . . is regulated mainly 

 with reference to the thickness of the stem" seems iomewhal 

 self-evident, as a large leaf must have a large stem to carry it, 

 may be seen by comparing the slender shoot of a Deodar 

 with a cabbage-stalk ; but he adds : " Thesize once determined 

 exercises much influence on the form." This is a deduction 

 which seems to require ■verification. Sir John gives the area of 

 a beech-leaf as about 3 square inches, but the form remains the 

 same whatever the size. Size rather depends on vigorous 

 growth, as in the following instances : Populus a'ba leaves on a 

 vigorous basal shoot were 64 ■ 3J inches, the diameter of the 

 shoot being £ inch ; on the upper branches of the same tree 

 many leaves were only i-i to 2i inches long, the diameter of the 

 shoot being also \ inch. Similarly growing oak leaves of the 

 same shape were 6x3 inches and 2 x \ inches respectively. 

 An Aucuba japonica bore rounded leaves on a basal shoot 4 X l\ 

 inches, but those on the stem were 4 X 1 inch. In this case, as 

 in other plants with (normally) dimorphic leaves, as ivy, it is 

 difficult to see what connection there is between size and form. 

 Indeed leaves of every degree of superficial area can be found 

 amongst the lobed ones on the climbing stem of ivy, and the 

 entire ones of the flowering branch. Sir John adds that "the 

 form of the inner edge [of the beech] . .'. decides that of the 

 outer one." He does not seem to have verified this deduction. 

 The two edges are symmetrical in this leaf, but they are not so 

 in the elm and iime. How will the inner edge explain the cause 

 of their obliquity ? If, however, the buds of the lime be exa- 

 mined, a more probable cause (as it seems to me) will be dis- 

 covered in the conditions of development. He describes the 

 Eucalyptus, when young, as having "horizontal leaves, which in 

 older ones are replaced by scimitar-shaped phyllodes." Bentham 

 and Hooker say of Eucalyptus : " Folia in arbore juniore ssepe 

 opposita, in adulto pleraque alterna," but makes no mention 

 of phyllodes. Speaking of evergreen leaves, he says : " Glossy 

 leaves have a tendency to throw [snow] off, and thus escape, 

 hence evergreen leaves are very generally smooth and glossy." 

 This sentence appears to imply that such leaves are glossy 

 in anticipation of snow ! a deduction which certainly re- 

 quires verification. Again : " Evergreen leaves often have 

 special protection ... by thorns and spines. Of this the 

 holly is a familiar illustration ; and it was pointed out that 

 in old plants above the range of browsing quadrupeds, the 

 leaves tend to lose their spines and become unarmed." The 

 inference the reader draws from this is that when the holly 

 grows out of reach of browsing animals it Ins no necessity to 

 produce prickly leaves, and so changes them accordingly, 

 thereby implying that unarmed leaves were in some way prefer- 

 able. This is another instance of deductive reasoning which 

 requires verification, for it seems to be attributing to the holly a 

 very unexpected process of ratiocination ! But it is not at all 

 usual for hollies to do this. I have several from six to nearly 

 twenty feet high, and not one has borne an unarmed leaf. 

 Though my cows do not touch a holly hedge, yet one young 

 bush lately planted has taken their fancy, and they have bitten 

 it all to pieces. On the other hand one bush (in the garden), 

 a variety with unarmed foliage, occasionally throws out a 

 branch with prickly leaves, though the cows are not admitted 

 where it grow 5. 



"fleshy haves were principally found in hot and dry 

 countries, where this peculiarity ['./, J had the advantage of 

 offering a smaller surface, and therefore exposing the plant less 

 to the loss of water by evaporation." Surely the usual explana- 

 tion, that it is the thick cuticle which prevents rapid exhalation 

 is a better reason than Sir John's deduction from the small size 

 of the leaves? Speaking of aquatic plants, he sa\ 

 submerged "cut up " leaves of such plants presents a greater 

 extent of surface;" and adds thai "such leaves would be 

 unable to support even their own weight, much less to resist 

 any force, such as that of the wind.." 1 1 be gl id to know 



it he ha- verified tile first statement by actual measurements; 

 for an 1) priori assumption leads one to fancy that a complete leaf 

 would have a greater surface than one represented by its ribs 



and veins only. With regard to the second and third statements 

 a " natural experiment " completely refutes his deduction, for I 

 know a place where a small pond dried up last summer, and a 

 large portion of the ground was covered with a dense velvet-like 

 carpet, composed of the erect filiform branchlets of the "cut-up" 

 leaves of Ranunculus aquatilis, which had become modified by 

 their new medium, and perfectly adapted to enjoy an aerial 

 existence. 



In offering these few criticisms for Sir John Lubbock's con- 

 sideration, I would venture to remark that he seems to have 

 followed to 1 closely in the deductive methods of another writer 

 on leaves, and which called forth the following remark from 

 Prof. Lankester : — [He] "gives us hypotheses, suppositions with 

 insufficient evidence, and deductions from the generalisation of 

 Evolution, but he is relatively deficient in 'verification'" 

 (Nature, vol. xxviii. p. 171). George Henslow 



Drayton House, Ealing 



The Fall of Autumnal Foliage 

 Mr. Fraser alludes to "the unpursued inquiry into the 

 came of leaves falling in autumn" (NATURE, February 26, 

 p. 38S), and I do not find it mentioned in Sach's " Text Book " ; 

 bul I Ir. :\ isters, in Henfrey's " Elementary Course of Botany," 

 fourth edition, p. 515, speaks of "a layer of thin-walled cells 

 being formed across the petiole," but does not say whence this 

 layer is derived. Duchartre, however, gives a pretty full account 

 ol'opinionsupto 1877 ("El. de Boh," deux. ed. p. 443), which he 

 reduces to two, vi.-. Schacht's, who attributes it to a growth of 

 periderm, and that of Mollis, who recognises a special layer which 

 he calls couche separatrice, considering the peridermic layer as being 

 often, but not always formed. Subsequently, M. Ledgeganck 



examined different plants and corroborated Schacht in regarding 

 the periderm as the cause pridisposante, and cold to be the cause 

 efficiente, which contracts " Ie tissu de la base du petiole, 

 spongieux, aere, elastique a un degre beaucoup plus considerable 

 que celui du coussinet." From my own observations on the 

 horse-chestnut, ash, &c, it appears to be in these clearly a con- 

 tinuation of periderm produced by the phellogen of the branch, 

 which invades the base of the petiole, till it meets in the middle, 

 cutting right through the fibro-vascular bundles of the petiole. 

 As this suberous layer dies, the leaf necessarily falls off. But as 

 long as a leaf is in vigorous health it would seem to resist this 

 invasion, and last longer, as do evergreens. I inclose a figure 

 I possess of a slide showing the process in the horse-chestnut. 

 Drayton House, Ealing George Hensi.ow 



Forest-Trees in Orkney 



In Nature of February 26 (p. 3SS) Mr. A. T. Fraser says 

 that "a peculiarity of Caithness and the Orkney and Shetland 

 Islands is that no forest-trees can be got to grow," and he pro- 

 ceeds to explain this by the preponderance of polarised light. 

 As far, at least, as Orkney is concerned, I am prepared to rebut 

 this calumny. It is true that forest-trees are not the striking 

 feature of the Islands, but they do occur. At Binscarth, be- 

 tween Kirkwall and Stromness, there are willow, ash, sycamore, 

 and Scotch fir. They require to be protected — from the wind, I 

 presume, and not from the light — by hedges of bour-tree (elder). 

 In the street at Kirkwall itself there is a fair-sized sycamore. 



Trinity College, Cambridge James Currie 



V"i . 1; Indian correspondent, Mr. A. T. Frazer, can hardly 

 be acquainted with the primitive jungles of Southern India, or 

 he would have observed that there, at one and the -ame time, 

 the aspect of all the four seasons is displayed in the vegetation. 



