218 



KNOWLEDGE 



[Jan. 13, 1882. 



the young (lowoi-s when tln'y arc ready to sprout. So the 

 moment a littlf warm wcatlier arrives, the liud l>e<j;iiis to 

 start into lite, and is supplied with food from the starcli 

 laid by in the root, as well as from the constant gains of 

 the ever-husy leaves. All annual plants have to grow from 

 the seed in a .sin^Je season, and they have to produce a 

 large numher of leaves ))efor(! they have digested food 

 enough in these their expanded stomachs to feed the future 

 rtowers and seed ; so that they cannot hegin blossoming 

 till comj^aratively late in the season. But the daisy, being 

 a perennial, with slightly starthy root and practically 

 persistent foliage, gets the .start of them from the 

 beginning, so a.s to put forth its flowers at the earliest 

 possible moment. 



And now let me look briefly at tliis flower itself. It is 

 made up, as everybody knows, of two parts. The centre 

 or disk is yellow, while tlie outer r.ays are white. But if 

 one pulls it to pieces, one sees that the disk is really com- 

 posed of many separate little golden bells, eacli one some- 

 thing like a harebell on a very small scale. The daisy 

 head, in fact, is not one flower, but a whole lot of distinct 

 flowers crowded together into a single truss. Taking one 

 of the little central j'ellow bells in detail, I find that its 

 petals are not separate, as in the biittercup, Imt are aU 

 united together into a long tube. The ancestors of the 

 daisy had doubtless ages ago five distinct petals, like those 

 of the buttercup ; but at some time or other these petals 

 showed a tendency to coalesce, and as this tendency 

 proved useful to tlie plant, by more certainly securing its 

 fertilisation liy insects, it rapidly grew through survival of 

 the fittest into a fixed habit, not only of the daisy, but of all 

 the great group of flowers to which it belongs. The reason 

 why the tubular shape is more useful than the arrangement 

 with five spreading petals becomes clear enough if we re- 

 collect that the insect has to thrust his proboscis down to 

 the bottom of the tube, past the pollen-bearing stamens and 

 the sensitive pistil, in order to reach the tiny drop of honey 

 concealed within. In doing .so, a little of the pollen natu- 

 rally adheres to his proboscis, aiid is then brushed olT 

 against the sensitive surface of the next blossom which he 

 visits, so as thus to impregnate and fertilise its seed. To 

 this day, however, the daisy still retains a reminiscence of 

 the distant period when it possessed fi\e separate petals ; 

 for each of the central florets has a vandyked edge of five 

 points, these points being the last representatives of the 

 original distinct flower leaves in its remote progenitors. 



The tubular arrangement is common to many flowers 

 besides the daisy family ; but the daisies and their allies have 

 carried their development one step further than the rest, for 

 they have learnt to collect several tiny blossoms together into 

 a single compact liead, and thus to bid foi- the attention of 

 insects far more powerfully than they could do in single 

 display. More than that, in tin; dai.sy itself, and one or 

 two others of its near relations, the outer florets of each 

 head have become flattened into long ornamental rays, so 

 as to play the part of petals tf» the compound group. In 

 this way they make the little bunch very noticeable to all 

 passing insects. The ray florets, when closely examined, 

 look like tube^ split down one sid(' and opened out, so as 

 to produce as much show as possil)le. They are the attrac- 

 tive part of the flower-head, and they do little acti\e work 

 themselves, having no stamens and no pollen,but laying them- 

 selves out mainly to look pretty alone. For tiiis purpose they 

 are coloured white, with |)inky tip.s, instead of lieing yellow, 

 like the central florets. Yet, of course, the v\hole plant is 

 ultimately benefited by this ari-angement, because the 

 insects are thus induced to visit the entire little colony at 

 once, and Viy carrying pollen from one floret to another, to 

 fertilise the whole row of yellow bells then open. For if 



you look int<'ntly into a daisy, you will see that it does not 

 open all over at the .same time, but Ixrgins opening from the 

 idge, and gradually proceeds towards the centre ; so that 

 in most daisies yo\i will find a row or two of over-blown 

 florets outside, a row just open or opening half-way through, 

 and a lot of unopened litth- buds in the very middle. 

 ]Joubtles.s, this arrangement also conduces to the good of 

 the plant, by ensuiing the highest and liest sort of cross- 

 fertili.sation — that which is obtained by impregnating the 

 blossoms of one individual with pollen brought from those 

 of another. 



PRECESSION OF THE EQUINOXES. 



By the Editor. 



THE precession of the equinoxes is. properly speaking, 

 the observed motion of the ]>oints oii the earth's orbit, 

 where she is when the sun pa.sses from north to south, or 

 from south to north of the celestial equator, these points 

 moving always in a direction contrary to that in which the 

 earth circles round her orbit, so that they travel backwards. 

 As precession really means going forwards, it may seem a 

 little strange that this travelling back%vardsof the equinoctial 

 points should have received such a name. But, as a 

 matter of fact, the motion of these points (which are those 

 where spring and autumn begin) in a direction opposite to 

 that of the earth's motion, causes them a.s it were to meet 

 the earth, shortening the time she takes in reaching them, 

 so that the beginnings of spring and autumn precede the 

 epochs at which otherwise the}' would have occurred. 



But usually the term " precession " is understood to 

 include all the circumstances on which the observed change 

 depends, and writers will often speak of the precessional 

 reeling of the earth. The reeling itself is, of course, not 

 precessional, it is but the cause of precession. 



The change in the position of the points where spring 

 and autumn begin was first discovered by Hipparchus, 

 though Ptolemy usually gets the credit of it. It was 

 found that the point of the ecliptic where the sun is when 

 spring begins — we may say when the year of seasons 

 begins — is slowly moving backwards among the zodiacal 

 constellations. It was once in Taurus, which was spoken 

 of even by Virgil (long after the relation had ceased to 

 exist) as opening the year with its golden horns. Then it 

 passed to Aries, thence to Pisces, in which constellation it 

 is now. 



The effects thus produced in the position of the celestial 

 equator, poles, kc, will occupy us hereafter. For the 

 present we wish to consider their cause, or, rather, first we 

 wish to describe what is the actual motion of the earth to 

 which they are due, the real cause of precession being 

 the forces which cause the eartli to move in the particular: 

 manner in question. 



Let us for the moment leave out of account the motion 

 of the earth round lier axis, and regarding her centre as at 

 rest, h^t us consider wliat is the real nature of that motion 

 of hers which causes precession. 



Observation shows that the plane of the ecliptic, in 

 which the earth travels, is almost unchanging in position. 

 The course among the stars along which the sun (if we 

 could see the stars when he is shining) would appear to 

 travel, is almost exactly the same now as it was in the 

 time of Hipparchus. Again, the inclination of the t>arth's 

 polar axis to the plane in which she travels varies very 

 slightly (the variation shall presently be described, and its 

 physical cause indicated). 



But the line in which the plane of the earth's equator 



