82 



NATURE 



[May 2 2, 1890 



oath of the oak, but overlooked the mistletoe. She 

 thought, however, and the divinities thought, that she 

 had successfully accomplished her mission, and th;it 

 Balder had received the gift of immortality. 



One day, thinking Balder proof, they amused them- 

 selves by shooting at him, posting him against a holly. 

 Loki tipped an arrow with a piece of mistletoe, against 

 which Balder was not proof. This unfortunately pierced 

 him to the heart, and he fell dead. Some drops of his 

 blood dropped on the holly, which accounts for the red- 

 ness of the berries ; the mistletoe was so grieved that 

 she has ever since borne fruit like tears, and the crow, 

 whose form Loki had taken, and which till then had been 

 white, was turned black. 



This pretty myth accounts for several things, but is 

 open to fatal objections. You will judge whether I am 

 more fortunate. In the first place I need hardly observe 

 that the forms of leaves are almost infinitely varied. To 

 quote Ruskin's vivid words, they "take all kinds of 

 strange shapes, as if to invite us to examine them. 

 Star-shaped, heart-shaped, spear-shapad, arrow-shaped, 

 fretted, fringed, cleft, furrowed, serrated, sinuated, in 

 whorls, in tufts, in spires, in wreaths, endlessly expressive, 

 deceptive, fantastic, never the same from footstalk to 

 blossom, they seem perpetually to tempt our watchfulness, 

 and take delight in outstripping our wonder." 



Now, why is this marvellous variety, this inexhaustible 

 treasury of beautiful forms ? Does it result from some 

 innate tendency of each species? Is it intentionally 

 designed to delight the eye of man ? Or has the form and 

 size and texture some reference to the structure and 

 organization, the habits and requirements, of the whole 

 plant ? 



The leaf, although so thin, is no mere membrane, but is 

 built up of many layers of cells, and the interior com- 

 municates with the external air by millions of little 

 mouths, called stomata, which are generally situated on 

 the under side of the leaf. The structure of leaves varies 

 as much as their forms. 



It is, of course, principally in hot and dry countries that 

 leaves require protection from too much evaporation. 



The surface is in some cases protected by a covering of 

 varnish, in others by saline or calcareous excretions. In 

 others, again, the same object is attained by increased 

 viscidity of the sap ; in some, the leaves assume a vertical 

 position, thus presenting a smaller surface to the rays of 

 the sun. In other cases the leaves become fleshy. Woolly 

 hairs are also a common and effective mode of protection. 

 The plants of deserts are very frequently covered with a 

 thick felt of hair. Some species, again, which are 

 smooth in the north tend to become woolly in the south. 

 Species of the cool spring again tend to be glabrous. 

 The uses of hairs to plants are indeed very various. 

 They serve, as just mentioned, to check too rapid evapora- 

 tion. They form a protection for the stomata or breath- 

 ing holes, and consequently, as these are mainly on the 

 under side of leaves, we find that when one side of the 

 leaf is covered with white felted hairs, as the white poplar, 

 this is always the under size. 



In other cases the use of hair is to throw off water. In 

 some Alpine and marsh plants this is important. If the 

 breathing holes became clogged with moisture — with fog, 

 for instance, or dew — they would be unable to fulfil their 

 functions. The covering of hair, however, throws off the 

 moisture, and thus keeps them dry. Thus these hairs 

 form a protection both against too much drought, and 

 too much moisture. 



Another function of hairs which cannot be omitted is 

 to serve as shades against too brilliant light, and too 

 much heat. Again, hairs serve as a protection against 

 insects, and even against larger animals. The stinging 

 hairs of the common nettle are a familiar example, and 

 coarse woolly hairs are often distasteful to herbivorous 

 quadrupeds. 



NO. 1073, "^'<^^L. 42] 



Ueciducus leaves especially characterize the compara- 

 tively cool and moist atmosphere of temper<-ite regions. 

 For different reasons evergreen leaves become more 

 numerous in the Alps and in the tropics. 



In the Alps it is necessary for plants to mnke the most 

 of the short summer. Hence, perennial and evergreen 

 species are more numerous in proportion than with us. 

 Everybody must have noticed how our trees are broken 

 if we have snow early in the season and when they are 

 still in leaf. 



The comparatively tough and leathery leaves, such as 

 those of the evergreen oak and olive, are protected 

 against animals by their texture, and often, as in the holly, 

 by spines ; they are better able to resist the heat and 

 dryness of the south than the comparatively tender leaves 

 of our deciduous trees, which would part too rapidly with 

 their moisture. It is perhaps an advantage to evergreen 

 leaves to be glossy, because it enables them better to 

 throw off snow. Moreover, their stomata are often placed 

 in pits, and protected with hair, which prevents too rapid 

 evaporation. The texture and structure of leaves is indeed 

 a wide and very interesting subject, but to-night I must 

 confine myself to the shape. 



It is impossible to classify plants by the form of the 

 leaf, which often differs greatly in very nearly allied 

 species. Thus the common plantain of our lawn {Plantago 

 major) has broad leaves, P. lanceolata narrow ones. The 

 width or narrowness of leaves depends on various con- 



i siderations. In herbaceous and stalkless plants, such as 

 the plantain, prostrate leaves tend to be broad, those 



j which are upright to be narrow. Thus, grasses, for in- 



! stance, have more or less upright narrow leaves. 



j In other cases the width is determined by the distance 

 between the buds, and in others again by the number of 

 leaves in a whorl. 



Cordate and Lobed Leaves. 



Among broad leaves we may observe two distinct 

 j types, according as they are oval or palmate. Monocotyle- 

 donous plants, such as grasses, sedges, lilies, hyacinths, 

 very generally have upright and narrow leaves. When 

 they are wider, as, for instance, in the black bryony, this 

 is mainly at the base, where, consequently, the veins are 

 further apart, coming together again towards the apex. 

 This we are tempted therefore to regard as the primitive 

 type of a broad leaf. 



There is, however, a totally different one, where the 

 leaf is palmate, like a hand, widening towards the free 

 end. Here the veins pursue a straight, diverging course ; 

 and as they not only serve to strengthen the leaf, but 

 also to carry the nourishment, this is doubtless an ad- 

 vantage. Another reason perhaps for this arrangement 

 is found in the fact that these leaves are generally folded 

 up, like a fan, while they are in the bud. 



I have elsewhere dwelt on the case of the beech, and 

 perhaps I may briefly refer to it again. The weight of 

 leaves which a branch can carry will of course depend on 

 its position and strength. The mode of growth of the 

 beech and the hornbeam are very similar, but the twigs 

 of the latter are slenderer, and the leaves smaller. If we 

 cut off a beech branch below the sixth leaf we shall find 

 that the superficial leaf area which it carries is about 18 

 square inches. But in our climate most leaves are glad 

 of as much sunshine as they can secure, and are arranged 

 with reference to it. The width of the beech leaves, 

 about 1 1 inch, is regulated by the average distance 

 between the buds. If the leaves were wider they would 

 overlap. If they were narrower there would be a waste 

 of space. The area on the one hand, and the width on 

 the other, being thus determined, the length is fixed, 

 because, to secure an area of 18 inches, the width being 

 about 1 1 inch, the length must be about 2 inches. This, 

 then, explains the form of the beech leaf. 



Let us apply these considerations in other cases. I 



