13C 



♦ KNOWLEDGE ♦ 



[Aug. 31, 1883. 



good reason, that they want to use the ends of their leaf- 

 stalks as tendrils to fasten them firmly on their unwilling 

 hosts. The vetches are not quite such highly-developed 

 plants as the true peas, and they supply us accordingly 

 with some very interesting gradations in this direction. 

 There is one rather uncommon British form ( Vicia orobus), 

 remarkable for its stem being more erect and its habit less 

 climbiug than in other vetches ; and this form has a 

 terminal leaflet to all the ujiper leaves, though in 

 the lower ones the common leaf-stalk ends instead 

 in a very rudimentary tendril, represented merely by 

 a short point. In another slightly more advanced type 

 (Vicia tetrasjyeriiia) there is always present a terminal 

 tendril instead of the last leaflet ; and sometimes this 

 tendril is branched, that is to say in other words, a pair of 

 lateral leaflets have also been reduced to naked twining 

 leaf-stalks. In our other vetches, the tendrils are almost 

 always branched, and one very developed type (Vicia 

 hithynica), confined in Britain to the West Country, has 

 only one pair of leaflets left. Of course, as these two 

 have to do all the work, they have grown comparatively 

 large and long, while in the other vetches, where the 

 leaflets are many, they are usually small and broad. The 

 true peas, forming the genus Lathyrus, show us a still 

 further progress in the same direction. We have one very 

 rare British species (it occurs with us only in Perth- 

 shire and Forfarshire, and is everywhere a dying 

 mountain type) which resembles Vicia orubus in having 

 a good many leaflets on each leaf, and in the occasional 

 reappearance of the terminal leaflet in the upper foliage. 

 But most of our peas have very few leaflets — indeed 

 sometimes two pairs, and sometimes only one, all the rest 

 being converted into tendrils to aid the climbing habit. At 

 the same time the work of foliage is thrown a good deal 

 on the stipules, which grow out accordingly into large, flat, 

 leaf-like organs, far more conspicuous in the garden pea 

 than the true leaflets, while the common leaf-stalk also 

 shows a tendency to be winged or expanded at the side, 

 which is very noticeable in the " everlasting pea " of our 

 flower-beds and trellises. These two last tendencies find 

 their final outcome in two English wild peas which have 

 no real leaflets at all, having entirely substituted for them 

 these auxiliary foliar organs. One is the so-called yellow 

 vetohling (Lathyrus aphaca), a weed of cultivation which 

 appears very occasionally in the cornfields of our southern 

 counties ; it has the two stipules immensely enlarged into 

 what looks like a pair of big opposite heart-shaped leaves, 

 with only a slender branching tendril between them to 

 represent the original leaf-stalk and leaflets. The other is 

 the grass-pea (Lathi/rus nlssolla) a pretty, grass-like plant, 

 with beautiful palered flowers ; it grows among tall grasses 

 on the borders of fields, and has been compelled by its 

 situation to imitate the shape of the surrounding blades, 

 which it does by flattening out the common leaf-stalk into 

 a long narrow ribbon, without stipules, leaflets, or even 

 tendrils, save iia the form of a tine point. The everlasting 

 pea, and some south European forms, give us a good idea 

 of the stages bj' which this curious transformation has 

 been efi'ected. The final result is that the grass-pea looks 

 in foliage almost like a grass or sedge, and does not in any 

 way recall its real ancestry from a highly compound 

 pinnate leaf like that of sainfoin or Hippocrepis. 



*^* TuE jiicture of the Whirlpool Rapids promised for 

 last number of Knowledge will appear next week. It 

 presents the Rapias as shown in an instantaneous photo- 

 graph, taken at a moment when the collision of the water 

 particles, so graphically described by Professor Tyndall, 

 had just taken place. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henry J. Slack, F.C.S., F.R.M.S. 



AMONG the objects mentioned in the last paper as afford- 

 ing beautiful specimens of plant- hairs strengthened by 

 silex, was the cuticle of Dmlzia scahra leaves. It is not 

 possible to make good preparations of this cuticle by 

 mechanical means, but it is easily effected by chemical 

 agency, and a similar plan is useful in a variety of other 

 cases. A perfectly sound well-grown leaf of Dcutzia scahra 

 should be cut into pieces about one-third of an inch square, 

 and the thick mid-rib rejected. Pieces of this size will lie 

 at the bottom of a large test-tube, six inches long and one 

 inch wide. If the experimenter is not provided with any 

 apparatus to hold such a tube, one can easily be made by 

 sticking a piece of stout wire eight or nine inches long into 

 a small square block of wood, and bending the top of 

 the wire into a ring just wide enough to hold the 

 tube by its projecting rim. The ring should be turned on 

 one side, so as to keep the tube in a slanting position. 

 This being arranged, pour some nitric acid into the test-tube, 

 so as to cover the bits of leaf and occupy a space of about 

 one inch in height. A spirit-lamp should be held under 

 the tube, and the acid made quite hot. Small crystals of 

 chlorate of potash should then be dropped in, one or two 

 at a time. They excite a brisk efl'ervescence. If added too 

 quickly, or the acid is heated too highly, the mixture swells 

 up suddenly, and may even boil over. This must be 

 avoided, as a violent treatment of the leaf tears the cuticle 

 into minute fragments, and from these no fine slides can be 

 prepared. If the frothing is too slow, a little more heat 

 should be applied, and the dropping in of the chlorate of 

 potash continued until nothing is seen but a collection of 

 white films floating on a bright yellow fluid. The tube 

 must now be allowed to cool, and then with a little 

 dexterity all the yellow fluid can be poured off, and the 

 films allowed to remain. They must be washed several 

 times by filling the tube with plain clear water and pouring 

 it oflT again. All these things must be done gently, so as 

 not to tear the films. 



Those accustomed to chemical operations will not need 

 more instruction, but for beginners further information is 

 necessary. The process is one of oxidising, or burning 

 away, all but the leaf cuticle. The oxygen is applied at the 

 expense of the chlorate of potash and of the nitric acid, 

 and as soon as the action goes on briskly, the latter gives 

 forth orange fumes of nitric-oxide gas.* This is not only 

 very unpleasant, but extremely unwholesome to breathe. 

 A good mouthful of it, even when largely mixed with 

 common air, is very suffbcatin" and highly injurious to 

 the lungs. The fumes should be allowed to go up 

 a chimney or out of window. The operator should 

 notice which way the air-currents carry them, and 

 keep, as sailors say, to the leeward. Beginners should 

 make their first experiment on a smaller scale than has 

 been mentioned. Nitric acid burns holes in clothes and 

 stains the skin yellow. Having completed the process as 

 described, the films may be kept in a small bottle, with a 

 little water and a few drops of alcohol. To prepare a slide, 

 take up a few films on a needle and drop them into a small 

 shallow saucer of clear water ; or, if a very small quantity 

 is on the needle, it may be placed on the centre of a glass 

 slide in a water-drop. The first plan is the best, as most 

 certain to remove any chlorate of potash that may not have 



* When nitric oxide meets common air, it instantly seizes its 

 oiiTgcn and becomes the orange gas nitrous acid, which is readily 

 absorbed by water and is highly corrosive. 



