Aug. ;31, 1883.] 



• KNOWLEDGE ♦ 



129 



V AN IlLLLSTLRATED 



" MAGAZINE OF^IENC 

 i PlainlyWorded -£xactlYDescribed 



LONDON: FRIDAY, AUG. 31, 1883. 



Contents 



PAGE 



A Naturalist's Tear. Wild Peas. 



By Grant Allen 129 



Pleasant Hours with the Microscope. 



ByH. J. Slack 130 



Tricycles in 1833 ; Small Wheels v. 



Large Wheels. By John Browning 131 

 The Amateur Electrician. (Illm.) 133 

 Laws of Brirhtness. IX. (Ilhu.) 



By E. A. Proctor 133 



The Chemistry of Cookery. XVII. 



By W. Mattieu Wmiams 135 



Evolution of Human Physiognomy. 



{lllus.) By E. D. Cope 138 



OP No. 96. 



PAGB 



The Morality of Happiness : Evo- 

 lution of Conduct. II. By Thos. 



Foster 133 



Pretty Proofs of the Earth's Hotun- 



dity. {lUm). By R. A. Proctor 139 

 Punctuation and Printers. By Sir 



Edmund Beckett 140 



The Face of the Sky 141 



Correspondence : Luminous Riiig — 

 Flight of a Vertical Missile, ic. 141 



Our Mathematical Column 143 



Our Whist Column 143 



Our Chess Column 144 



A NATURALIST'S YEAR. 



By Grant Allen. 



XX. — WILD PEAS. 



AMONG the hedgerows here, on the side of a deep 

 Surrey lane, the lithe curling stems of the common 

 English yellow pea clamber in their usual straggling 

 fashion over the loose outlying sprays of bramble and haw- 

 thorn bushes. Looking close into the hedge, you can see 

 that they festoon themselves by means of their tight- 

 twisted tendrils, which wind and insinuate their graceful 

 coils round and round the successive supports by whose 

 aid they struggle gradually upward towards the air and 

 light. Strange as it sounds to say so, these thin spiral 

 tendrils are in reality metamorphosed leaves, or perhaps one 

 ought rather to put it, tlie abortive footstalks of un- 

 developed leaflets. Pull off one of the whole leaves 

 from the stem, and you will see it consists of a 

 central midrib or common leaf-stalk, with two little 

 arrow-headed stipules at its base, and one pair of 

 very narrow leaflets half-way up ; but the end of the 

 leaf-stalk bears no more leaflets, and in their place it ends 

 in a branched tendril, which really represents the altered 

 and modified remnant of the remaining blades. See here, 

 by the side of the yellow peas I have picked for compari- 

 son, two other leaves belonging to the same peaflower tribe; 

 one is the pretty purple-tufted vetch {Viciti cracca), a 

 common wayside plant over all Britain ; the other is the 

 bright golden Hippocrepis, abundant in chalky pastures 

 and on limestone banks in Southern England. You will 

 observe that the tufted vetch has several pairs of leaflets to 

 each leaf , arranged in couples on opposite sidcsof the common 

 stalk, instead of one pair only, as in the yellow pea : but 

 at the end, it terminates in a branched tendril of much the 

 same character as that of its yellow ally ; while in the 

 Hippocrepis each leaf-stalk, besides bearing some four or 

 eight pairs of lateral leaflets, ends also in a single terminal 

 leaflet, which occupies the place taken by the tendril both 

 in the tufted vetch and in the yellow pea. These two 

 cases help us to understand the line of development by 

 which the leaflets at the end of the stalk in the most 

 clamliering species of poailowers have been gradually meta- 

 morphosed into twining tendrils. If wo take a rapid glance 

 at the general type of foliage in the entire family, and at 



the special modifications which that type undergoes in 

 adaptation to the varying environments of diverse divergent 

 forms, we shall still better understand the nature and origin 

 of these peculiar elongated leaf-organs. 



The original and central peaflower leaf, from which all 

 the other leaves of the group are derived by suppression or 

 alteration of particular features, consists of a long central 

 leaf-stalk, terminating in a single leaflet, and with other 

 similar leaflets arranged in pairs down the opposite sides. 

 This form of leaf is very well seen in Hippocrepis ; and 

 among other English plants of the family which e.xhibit it 

 equally well, I may mention the pretty little bird's-foot 

 {Ornithopus jjerjnuillus), the common lady's-fingers {An- 

 thyllis vulneraria), and the beautiful pale pink sainfoin 

 (Onobri/chis saliva), if not indigenous, at least now fully 

 naturalised in the southern and eastern counties. That 

 this long pinnate type of leaf is the original one we can 

 gather both from the ease with which all the other forms 

 can be derived from it, from their occasional relapse into 

 it, and from the fact that it also appears in some of the 

 simplest roses, of which the peaflowers are genetically a 

 slightly specialised oflshoot. This central type of leaf occurs 

 mainly in those peaiiowers which grow in open ground, 

 where all the blade is freely e.xposed to air and sunlight. 



The easiest variation upon the central type is found in 

 certain divergent peatlowers, like the clovers, which inter- 

 mingle freely with the grasses in meadows, and have to 

 compete with them for their fair share of space and sun- 

 shine. In these cases the leaflets on the lower part of the 

 leaf-stalk are never developed, because they could do no 

 good to the plant ; they would be over-shadowed by the 

 grasses and other tall weeds on every side of them. Hence 

 they would naturally grow smaller and smaller by disuse 

 with each generation ; while, at the same time, those 

 plants which showed the greatest tendency to get rid 

 of them, and to produce the three upper leaflets 

 only (the terminal one, and a single pair below it), 

 would be most highly favoured in the struggle for 

 existence, because they wasted none of their material in 

 places where it would be comparatively inefliective. So, in 

 the long run, the clovers have come to possess the familiar 

 trefoil type of leaf, consisting of three broad and expanded 

 leaflets, all equally exposed to the sunshine at the top of 

 their naked and elongated leaf-stalk. The common lotus 

 shows us an intermediate stage between this type and the 

 primitive pinnate form, for in that case there are five leaf- 

 lets, three arranged in a trefoil at the end of the stalk, and 

 two broad ones lower down, practically ururping the place 

 and function of the original stipules. Other English pea- 

 flowers of the trefoil group are lucerne, nonsuch, and the 

 other Medicagos, besides the rarer Melilotus and Trigonella. 

 Broom is a bushy example of the same sort ; but in it 

 the two lateral leaflets are often wanting, and only the 

 terminal one is developed. This last state, well adapted 

 for shrubs and bushes, has become habitual in the English 

 genistas, like petty whin ; but some exotic genistas, culti- 

 vated in our conservatories, still retain the trefoil type of 

 foliage. Furthest removed of all from the central type on 

 this line of development is the common gorse, where the 

 adult leaves are reduced to mere stiflP, simple prickles; but 

 even in this instance we have some faint memorial left of 

 the earlier habit ; for, as I have pointed out here already, 

 the young seedling gorse plants have trefoil leaves, which 

 only gradually merge as the shrub grows older, first into long, 

 thin leafy, blades, and finally into sharp and rounded bristles. 



On the opposite line of development, towards tlie peas 

 and vetches, we get this system of suppression among the 

 leaflets exactly re\ersed. The climbing types get rid, not 

 of the lowest, but of the terminal leaflets ; and for this very 



