Feb. 21, ISS5.] 



KNOWLEDGE 



173 



The pseudoniorphs — being harder than the surrounding 

 cla)- — then stand out boldly on the weatheitd surfaces. 

 No fossils occur in the Triassic marls, and we aie not sur- 

 prised at their absence, when we cons-idcr the impure state 

 of the water — inimical to life — in which these marls were 

 deposited, under conditions analogous to those which now 

 prevail in the Dead Sea, Lake Utah, itc. But the Keuper 

 sandstones afford a scanty fauna. From the old ballast- 

 cutting in this sandstone, in the Dane llill.<!, I obtained 

 numerous specimens of a little crustacean — Esllieria — 

 whose carapaces (all that remain) are exactly like a little 

 furrowed shell, together with some good iish-spiues. 



The average thickness of the "red rocks" of South 

 Leicestershire is from 500 to 700 feet, and tluy cover ovtr 

 with their undulations both the old slates of Chernwood, 

 and the softer shales, with their inter-bedded coal seams, 

 which form the eastern half of the Leicestershire coal- 

 field, and which are reached b}- shafts through the red 

 marls at Whitwick, Bagworth, Nailston, EUistowu, ic. 

 Roughly speaking, the River Soar forms the eastern 

 boundary of the Trias; for along the general line of its 

 coui-se from south to north the red marls disappear beneath 

 the stifi" clays of the Lower Lias, which form the famous 

 grass-lands and hunting fields of East Leicestershire. 



The Triassic marls dip to the east, and above them come 

 the Rhaetic bedt^, about forty feet in thickness ; they con- 

 sist of black and blue shales containing numerous fossils. 

 They mark a change iu the condition of things, due pro- 

 bably to a depression of the land-border of the Triassic 

 lake by which the waters of the exterior seas were allowed 

 to enter, bringing with them ne;v and numerous forms of 

 life. These Rhaetic beds cap the Spinney Hills, being well 

 exposed in the brick-pits near St. Saviour's Church. I de- 

 scribed their occun-ence here in the " Journal of the Geolo- 

 gical Society" for 1^73, and my search in the shales was re- 

 warded by many fossils, especially arare and beautiful starfish 

 then found for the first time in these beds. The visitor 

 who walks eastward from the Leicester (Midland) station 

 can examine within less than a mile both the Triassic and 

 Rhaetic sections of the Spinney Hills. If he continues his 

 walk for a mile and-a-half in the same direction, he will 

 find, on Crown Hill, some good sections of the Lower Lias 

 Limestones and Shales, which rest upon the Rhaetic beds, 

 forming the low hills between Leicester and the village of 

 Evington. 



There is an excellent section of the entire thickness 

 (-10 ft.) of the Rhaetics in the brickworks cIo.se to the new 

 railway station (L. it N.-W.) at Glen Parva, four miles 

 south of Leicester. 



{To he co7>tinued). 



FORMS OF LEAVES. 



A lECTCKE 



By Sir John Lubbock, Bakt., M.P., D.O.L., LL.D., 

 r.R.S., &c. 



G\ REA.TLY as we all appreciate the exquisite loveliness 

 r of flowers, it must be admitted that the beauty of 

 our woods and fields was even more due to the marvellous 

 grace and infinite variety of foliage. How is this inex- 

 haustible richness of forms to be accounted for 1 Does it 

 result from an innate tendency of the leaves in each species 

 to assume some particular shape ? Has it been intentionally 

 designed 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, firstly, the size of the leaf, we shall 

 find that it is regulated mainly with reference to the thick- 



ness of the stem, and that, when strict proportion is departed 

 from, the ditlerence can generally bo accounted for. 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. 



The size once doterniined exercises much inlluence on the 

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

 about three square iuchcs. The distance between the buds 

 is about 1.} in., 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 swell of the twig, while the 

 upper half follows the edge of the leaf above ; and the form 

 of the inner edge being thus determined, decides that of 

 the outer one also. In the lime the internodes 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, conseiiucntly, can carry a 

 larger leaf surface. But the distances between tho 

 buds are often little greater than those iu tho beech. This 

 determines, then, the width, and, by compelling the leaf to 

 lengthen itself, leads to the peculiar form which it assumes. 

 Moreover, not only Jo the leaves on a single twig admirably 

 fit one another, but they are also adopted to the ramifica- 

 tion of the twigs themselves, and thus avail themselves of 

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

 without large interspaces or much over-lapping. In the 

 sycamores, maples, and horse-chestnuts, the arrangement is 

 altogether different. The shoots arestifl' 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 o; these leaves is adapted to the mode of growth of, 

 and arrangement of, the buds iu the plants themselves. 



In the black poplar the arrangement of the leaves is 

 again quite diflerent. 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 con- 

 trary, they are very similar. The stomata, or breathing- 

 holes, moreover, which in the leaves of mcst trees are con- 

 fined to the under surface, are in this species nearly equally 

 numerous on both. The " compass " plant of the American 

 prairies — a yellow composite, not unlike a small sunilower 

 —is another plant with upright leaves, which, growing in 

 the wide open prairies, tend to point north and south, thus 

 exposingboth surfaces equally to the light and heat. It 

 was shown by diagrams that this position also aftected the 

 internal structure of the leaf. 



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

 and are long and linear, while in ths box they are further 

 apart and broader. In the Scotch fir the leaves are linear 

 and 1 .'; in. long, while in other pines— as for instance, the 

 Weyniouth — the stem is thicker and the leaves longer. 



In the plants hitherto mentioned, one main consideration 

 appears to be the securing of as much light as possible ; 

 but in tropical countries the sun is often too powerful, and 

 the leaves, 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 tho genus. Gradually, how- 



