39§ 



NATURE 



[Fed. 26, 1885 



provinces of China proper with the districts in India corre- 

 sponding nearly in physical characteristics and cultivable area, 

 and, summarising these computations, he found that, over a 

 total area of 1,500,650 square miles, the population, according 

 to this estimate from the Indian averages, would be 282,161,923, 

 or, say, 183 persons to the square mile, while the latest official 

 returns obtained from China show 349,885,386, or 227 inha- 

 bitants to the square mile. The general conclusion, he said, 

 might be that the latest Chinese returns, though probably in 

 excess of the reality, did not seem to be extravagant or in- 

 credible on the whole if tested by the known averages of the 

 Indian census. 



THE FORMS OF LEA VES » 



SIR JOHN LUBBOCIv said that, greatly as we all appre- 

 ciated the exquisite loveliness of flowers, we must ad- 

 mit that the beauty of our woods and fields was as much 

 due to the marvellous grace and infinite variety of foliage. How 

 is this inexhaustible richness of forms to be accounted for ? Does 

 it result from an innate tendency of the leaves in each species to 

 assume some particular shape ? Has it been intentionally de- 

 signed to delight the eyes of man? Or has it reference to the 

 structure and organisation, the wants and requirements of the 

 plant itself? 



Now, if we consider first the size of the leaf, we shall find ih.it 

 it is regulated mainly with reference to the thickness of the stem. 

 This was shown, for instance, by a table giving the leaf area and 

 the diameter of stem of the hornbeam, beech, elm, lime, Spanish 

 chestnut, ash, walnut, and horse-chestnut. When strict proportion 

 is departed from, the difference can generally be accounted for. 



The size once determined exercises much influence on the 

 form. For instance, in the beech the leaf has an area of about 

 3 square inches. The distance between the buds is about 1 .} inches, 

 and the leaves lie in the general plane of the branch, which 

 bends slightly at each internode. The basal half of the leaf fits 

 the swelf of the twig, while the upper half follows the edge of 

 the leaf above, and the form of the inner edge being thus deter- 

 mined decides that of the outer one also. In the iime the inter- 

 nodes are longer and the leaf consequently broader. In the 

 Spanish chestnut the stem is nearly three times as stout as that 

 of the beech, and consequently can carry a larger leaf surface. 

 But the distances between the buds are often little greater than 

 those in the beach. This determines, then, the width, and, by 

 compelling the leaf to lengthen itself, leads to the peculiar form 

 which it assumes. Moreover, not only do the leaves on a single 

 twig admirably fit one another, but they are also adapted to the 

 ramification of the twigs themselves, and thus avail themselves 

 of the light and air, as we can see by the shade they cast with- 

 out large interspaces or much overlapping. 



In the sycamores, maples, and horse-chestnuts the arrange- 

 ment is altogether different. The shoots are stiff and upright, 

 with leaves placed at right angles to the plane of the branch, 

 instead of being parallel to it. The leaves are in pairs, and 

 decussate with one another, while the lower ones have long 

 petioles, which bring them almost to the level of the upper 

 pairs, the whole thus forming a beautiful dome. 



For leaves arranged as in the beech, the gentle swell at the 

 base is admirably suited ; but in a crown of leaves, such as those 

 of the sycamore, space would be thereby wasted, and it is better 

 that they should expand at once, as soon as their stalks have 

 carried them free from the upper and inner leaves. Hence we 

 see how beautifully the whole form of these leaves is adapted 

 to the mode of growth and arrangement, of the buds in the 

 plants themselves. 



In the black poplar the arrangement of the leaves is again 

 quite different. The leaf-stalk is flattened, so that the leaves 

 hang vertically. In connection with this it will be observed 

 that, while in most leaves the upper and under surfaces are quite 

 unlike, in the black poplar, on the contrary, they are very simi- 

 lar. The stomata or breathing-holes, moreover, which in the 

 leaves of most trees are confined to the under surface, are in 

 in this species nearly equally numerous on both. The "com- 

 pass" plant of the American prairies, a yellow composite no! 

 unlike a small sunflower, is another plant with upright leaves. 

 which, growing in the wide open prairies, tend to point north 

 and south, thus exposing both surfaces equally to the light and 



1 Abstract by the Author of a Lecture delivered at the Royal Institute, 

 Feb. 13 by Sir John Lubbock, Bart., M.P., D.C.L., LL.D., F.R.S., &c. 



heat. It was shown by diagrams that this position also affected 

 tire internal structure of the leaf. 



In the yew the leaves are inserted close to one another, and 

 are long and linear ; while in the box they are further apart and 

 broader. In the Scotch fir the leaves are linear, and 1^ inch 

 long; while in other pines, as, for instance, the Weymouth, the 

 stem is thicker and the leaves longer. 



In the plants hitherto mentioned one main consideration ap- 

 pears to be the securing of as much light as possible ; but in 

 tropical countries the sun is often too powerful, and the loaves, 

 far from courting, avoid the light. The typical acacias have 

 primate leaves, but in most Australian species the true leaves 

 are replaced by a vertically flattened leaf-stalk. It will be 

 found, however, that the seedlings have leaves of the form 

 typical in ihe genus. Gradually, however, the leaf becomes 

 smaller and smaller, until nothing is left but the flattened leaf- 

 stalk or phyllode. In one species the plant throughout life 

 produces both leaves and phyllodes, which give it a very curious 

 and interesting appearance. In eucalyptus, again, the young 

 plant has horizontal leaves, which in older ones are replaced by 

 scimitar-shaped phyllodes. Hence the different appearances of 

 the young and old trees which must have struck every visitor to 

 Algiers or the Riviera. 



We have hitherto been considering mainly deciduous trees. 

 In evergreens the conditions are in many respects different. It 

 is generally said that leaves drop off in the autumn because they 

 die. This, however, is not strictly correct. The fall of the 

 leaf is a vital process connected with a change in the cellular 

 tissues at the base of the leaf-stalk. If the leaves are killed too 

 soon they do not drop off. Sir John illustrated this by some 

 twigs which he had purposely broken in the summer ; below 

 the fracture the leaves had been thrown off, above they still 

 adhered, and so tightly that they could support a considerable 

 weight. In evergreen trees the conditions are in many respects 

 very different. It is generally supposed that the leaves last one 

 complete year. Many of them, howei 1. attain 

 age : for instance, in the Scotch fir, three or four years ; in the 

 spruce and silver, six or seven ; in the yew even longer. It 

 follows from this that they require a tougher and more healthy 

 texture. When we have an early fall of snow our deciduous 

 trees are often much broken down ; glossy leaves have a ten- 

 dency to throw it off, and thus escape, hence evergreen leaves 

 are very generally smooth and glossy. Again, evergreen leaves 

 often have special protection either in an astringent or aromatic 

 taste, which renders them more or less inedible ; or by thorns 

 and -pines. Of this the holly is a familiar illustration ; and it 

 was pointed out tint in old plants above the range of browsing 

 quadrupeds, the leaves tend to lose their spines and become 

 unarmed. The hairs on leaves are another form of protection ; 

 on herbs the presence of hairs is often associated with that of 

 honey, as they protect the plants from the visits of creeping 

 insects. Hence perhaps the tendency of water species to become 

 glabrous, /"■ 'ygomun amphiblum being a very interesting case, 

 since it is hairy when growing on land, and smooth when in 

 water. Sir John then dealt with cases in which one species 

 mimics another, and exhibited a striking photograph ol a group 

 of stinging nettles and dead nettles, which were so much alike 

 as to be hardly distinguishable. No one can doubt that the 

 stinging nettle is protected by its poisonous hairs, and it is 

 equally clear that the innocuous dead nettle must profit by its 

 similarity to its dangerous neighbour. Other similar cases were 

 cited. 



He had already suggested one consideration, which in certain 

 cases determined the width of leaves ; but there were others in 

 which it was clue to different causes, one being the attitude of the 

 leaf itself. In many genera with broad and narrow-leaved 

 species, drosera and plantago, for instance, the broad leaves 

 formed a horizontal rosette, while the narrow ones were raised 

 upwards. Fleshy leaves were principally found in hot and dry 

 countries, where this peculiarity had the advantage of offering a 

 smaller surface, and therefore exposing the plant less to the loss 

 of water by evaporation. 



Sir John then passed to aquatic plants, many of which have 

 two kinds of leaves : one more or less rounded, which floats on 

 the surface ; and others cut up into narrow filaments, which 

 remain below. The latter thus presents a greater extent of 

 surface. In air, however, such leaves would be unable to sup- 

 port even their own weight, much less to resist any force such as 

 that of the wind. In perfectly sti'l air, however, for the same 

 reason, finely divided leaves may be an advantage, whereas in 



