April 1. 1901.] 



KNOWLEDGE. 



81 



Aspen leaf we notice that while the lower part of it is 

 circular in section, the part near the leaf is much 

 flattened, permitting free movement in the plane of the 

 leaf-blade. This, together with the position in which 

 the leaves are borne on the twigs, causes the leaves to 

 hang vertically. One result is that the light can stream 

 almost unbroken through the branches even to the 

 ground below, the wealth of foliage producing but a 

 faint tremulous shadow as the leaves rustle in response 

 to every breath of air. Well does Scott, seeking for a 

 simile, say in ilarmion: — 



" A'arinble as the shade 

 By tlie light quivering a*j)cn made." 



A peculiar point about these vertical leaves should be 

 noted. In discussing the question of plant-food in Know- 

 LEncE last year, Mr. Pearson explained the important 

 functions fulfilled by the leaves; how on the under side 

 of leaves are situated a myriad of tiny openings (^tonuita, 

 mouths) through which the plant absorbs carbon 

 dioxide from the atmosphere, and having taken from 

 it the carbon, liberates the oxygen, the stomata being 

 also used for the escape of the surplus water of the plant. 

 Now, the reason why these mouths are situated in most 

 plants on the under side of the leaves is no doubt be- 

 cause they are thus protected from cold and rain and 

 storm, and their work less interfered with. In the 

 Aspen, with its vertical leaves, either side of which is 

 eqtially exposed to atmospheric vagaries, there is 

 nothing to choose between the two sides as regards the 

 position of the stomata, and as a matter of fact, these 

 are equally distributed over both sides of the leaf. A 

 further modification of this kind we may find in plants 

 like the Water-lily, the leaves of which float on the sur- 

 face of water. Following out our line of argument, we 

 would expect to find the stomata confined to the njiper 

 side of such a leaf, so that they may be in contact with 

 the atmosphere, and this is exactly what we do find. 

 Plants whose leaves are all continually below the surface 

 of the water, such as the Water Lobelia and many Pond- 

 weeds, must perforce be content with obtaining the 

 carbon dioxide which they require from the siiiall 

 quantity of that gas which is to be found dissolved in 

 the water. 



The protection of leaves against various hurtful 

 agencies next claims our attention. The typical leaf 

 has its upper surface built of strong closely-placed cells, 

 to offer a stout resistance to rain and hail, and to ficst 

 or overpowering sun-heat. In hot dry weather, when 

 great evaporation is taking place, the plant can close up 

 all its stomata — shut down, so to speak, all the sluices 

 by which the water employed to convey dissolved salts 

 from root to leaf is allowed to escape, and thus retain 

 an abundant water-supply in spite of parching heat. 

 But in arid ground, such as sandy wastes or sea-beaches, 

 further protection against over-transpiration may be 

 desirable, and this is frequently effected by impervious 

 varnish-like layers on the upper surface of the leaves, or 

 by dense coverings of hairs. Layers of impermeable 

 corky cells in the epidermis or skin of the leaves are 

 also ' frequently to be found in plants liable to ex- 

 cessive transpiration. Such impermeable leaves are 

 beautifully developed in plants like the Stonccrops, 

 which, gi-owing in dry ground and on rocks, and being 

 liable to long-continued drought, store up in their leaves 

 a copious water-supply. Such reservoir-leaves are 

 greatly developed in the "plants of desert countries. Pro- 

 tection against the often fatal effect of frost is likev.ise 

 afforded by a thickening of the cuticle of leaves, and 

 especially by felt-like coverings of hairs. In some note- 



worthy cases protection against cold is effected by means 

 of movement on the part of the leaves. The most 

 familiar examples occurring among our native plants 

 are furnished by the trifoliate leaves of many of the 

 Clover family. As evening approaches, the Clovers and 

 their allies fold their three leaflets together by means of 

 an upward movement; the juxtaposition of the leaflets 

 retards loss of heat, and the vertical position which 

 they thus assume has the same effect, tending to check 

 the radiation of heat to the cold sky overhead. Tlie 

 Wood Sorrel, which, though of a quite different order, 

 has leaves which resemble those of the Clovers, effects 

 the same object by folding its leaflets Jounwdrfl^. 



Wet, which by lying on the leaves might hinder trans- 

 piration, must also be guarded against; a danger which 

 in many species is obviated by means of a waxy excre- 

 tion, especially on those parts of the leaves where the 

 stomata are situated; on which, as on an oily surface, 

 water will not lie. 



Another danger to which plants are exposed, and one 

 which we might think they would be powerless to meet, 

 is the attacks of browsing animals — animals of all sizes, 

 from minute insects up to gi-eat munching cattle. But 

 to note how perfectly such defence may be provided for 

 we need only look at our common Gorse, which boldly 

 invades the pasture, protected by its impenetrable 

 chevaux-de-frise. This plant, indeed, seems to have put 

 so much of its vital energy into the production of spines, 

 that it has none left with which to produce leaves, and, 

 as already remarked, the making of plant-food has to 

 be can'ied on by the green and much-branched stems. 

 The beautiful tribe of the Thistles naturally comes to 



Winter leaf-rosette of the Hemlock Stork's-bill. 



From a photograph ty Mr. H. J. Sf.vm,-juk 



our minds in this connection. Armed with innumerable 

 spines of the most exquisite structure, sliarper and more 

 delicate far than needles, the Spear Thistle and Marsh 

 Thistle raise their tall and graceful forms untouched 

 amid the close-browsed herbage, and without fear of 

 molestation — save from man, with his implements of 

 iron — open their flower-heads to the sun and the insects, 

 and scatter their numberless winged fruits to the wind. 

 In the Thistle the spines are borne alike on the stems, 

 leaves, and involucres or outer whorls of the heads of 

 flowers. The Holly is an interesting case. In low 

 bushes the edges of the leaves are provided with strong 



