426 



KNOWLEDGE. 



November, 1913. 



month later than the time that would give the best display. In 

 1852 it was on October 12th, or a fortnight before the time of 

 best display. It is fortunate that the conditions should be 

 good at two consecutive returns. There is little doubt that 

 the comet will be faintly visible to the naked eye at the time 

 of perihelion ; it will doubtless be visible in a binocular 

 throughout November. 



This is the fourth member of the Neptune comet family to 

 be observed at a second return. The others were Halley, 

 Olbers, Pons- Brooks. There are two others, seen in 1846 and 

 1847, whose returns are expected about 1921 and 1927. 

 Westphal's has much the shortest period of the family. Its 

 aphelion distance is thirty, exactly Neptune's distance, but 

 there is not a close approach to Neptune's orbit, owing to the 

 large inclination. The time of perihelion is five months 

 earlier than that predicted as the most probable by Hnatek 

 and Viljev. This illustrates the difficulty of accurate predic- 

 tion in a period of this length. 



THE PLANETARY DISTANCES.— I lately received from 

 Professor Lowell an interesting essay on the planetary 

 distances and their bearing on the question of the manner 

 of development of the system. He points out how often the 

 periods of two adjacent planets approximate closely to some 

 simple ratio. Thus : — 



Mercury to Venus \ 

 Jupiter to Saturn / 

 Venus to Earth 

 Earth to Mars 

 Saturn to Uranus .. 

 Uranus to Neptune 



= i 

 = I 



(A closer) 

 (A closer) 



He suggests that there is not only a coincidence but a law 

 here, and that " each planet has formed the next in order at 

 exactly one of these commensurable points, at the same time 

 displacing it slightly Sunward." 



He points out that the action of an outer planet on an inner 

 practically diminishes the Sun's mass and increases the period, 

 while the action of an inner one on an outer increases the 

 Sun's mass and diminishes the period, the second case being 

 the more effective. Thus a planet once formed tends to draw 

 neighbouring particles to itself by bringing their periods into 

 conformity with its own. 



He then proceeds to consider the effect of commensurability 

 of motion, showing that there is a tendency for particles to 

 swing about the commensurate position ; he suggests that in 

 time this leads to the building-up of a planet at this position. 

 The planets would thus have been formed from the inside 

 outwards, " each acting as a sort of elder sister in bringing up 

 the next." Jupiter is supposed to have been formed ante- 

 cedently to the commencement of this action, and Saturn, 

 Uranus, and Neptune to have been formed outside in 

 succession, each being drawn slightly Sunward from the point 

 of exact commensurability. Thus is explained the youthful 

 appearance and small density of Uranus and Neptune. Had 

 they started at the same time as Jupiter, their small size 

 would lead us to expect further development and greater 

 density. Saturn, on this view, is the youngest planet as 

 regards stage of development reached, which accords well 

 with the very small density and the presence of the ring. 



The four inner planets are supposed to have been formed 

 in a similar manner. The present condition of Mars is quite 

 consistent with an origin later than that of the Earth ; for in 

 spite of its small size it has evidently not yet reached its 

 " dead " stage. Professor Lowell points out that though 

 there are gaps in the minor planets at the distances cor- 

 responding with periods one-half, two-fifths, and one-third of 

 Jupiter's, yet the great bulk of them is concentrated near 

 these points. He suggests that we see here the same con- 

 gestion of matter as caused planetary aggregation elsewhere, 

 but in this case it proved abortive. 



The paper is an interesting attempt to give a physical 

 explanation of the existing planetary distances. He ventures 



to predict the distance 47-5 (period 328 years) for the planet 

 outside Neptune. From the analogy of the satellite system he 

 thinks it likely that it would have a large eccentricity and 

 inclination. This would make its discovery more difficult, 

 as a much wider zone would have to be swept. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S 



RECENT RESEARCH ON EMBRYOLOGY OF 

 ANGIOSPERMS.— {Continued from page 384). 



Schoute's work on the exact relationship of plerome and 

 periblem at the growing- point to the central cylinder and 

 cortex as differentiated in the older regions of the same axis, 

 whether stem or root, is very important. He accepts Van 

 Tieghem's definition of the stele as the solid cylinder of root 

 or stem enclosed within the endodermis. The endodermis 

 itself is considered as belonging to the cortex because in the 

 root its cells are opposite the radial files of the inner cortex, 

 and indeed form the inmost rank of these files ; this is assumed 

 to indicate a common origin by repeated tangential division. 

 The cells of the pericycle, the outermost layer of the stele, 

 alternate with those of the endodermis. As a rule there is no 

 corresponding radial arrangement in the cortical tissue of the 

 stem, but where such exists (as in the stem of Hippuris) the 

 endodermis is again included in it and terminates it. Schoute 

 in 1903 got precise results in species of Hyacinthus, 

 Helianthus, and Linum, in the roots of which the periblem 

 passed into the cortex, its inner layer becoming the 

 endodermis, and the plerome gave rise to the stele only ; but 

 owing to difficulties arising chiefly from the insertion of 

 leaves close up to the stem apex and displacements in the 

 original stem structure owing to this habit, Schoute found 

 definite results only in Hippuris, where the plerome gave rise 

 not only to the stele but also to the endodermis and to the 

 two or three inner layers of cortex immediately beyond it. 

 If Schoute's results are well founded the limit between 

 plerome and periblem does not correspond with that between 

 stele and cortex in the stem of Hippuris, and doubt is also 

 thrown on the assumption made by all previous observers that 

 rows of cortical cells arranged in radial files must be of 

 common origin. 



The stelar hypothesis is essentially an assertion of the real 

 homology between the vascular systems of stem and root 

 throughout all vascular plants. No difficulty arises so long as 

 we are dealing with roots only, or with the stems of those 

 vascular Cryptogams in which the vascular system is a closed 

 cylinder without gaps at the insertion of the leaf-traces : in 

 such plants the vascular cylinder is as well defined as in all 

 roots and can be described in the same terms. But the case is 

 quite different in the stems of Phanerogams, where apparently 

 the primary vascular cylinder is a system built up of leaf-traces 

 embedded in a parenchymatous matrix. The early anatomists 

 were faced at once by this problem in its crudest form, for they 

 began with the primary structure of the dicotyledonous stem, 

 and that of the root was not clearly understood until many 

 years later ; since they attempted to interpret it by reference to 

 the skeleton of the stem and in the same terms ; though there 

 is nothing in the anatomy of the root to correspond with the 

 leaf-trace, and the leaf-trace is the vascular unit of stem 

 structure in all Phanerogams. Even when the facts of root 

 structure were accurately known, the conception of the leaf- 

 trace bundle as the structural unit continued to be a stumbling- 

 block. Modern anatomy dates from 1871, when Van Tieghem 

 published the first of his series of memoirs in which the axial 

 core of the root was treated as equivalent to the whole system 

 of leaf-traces in the stem ; a conception which gained ground 

 from the first, and was popularised by the happy choice of 

 the term " stele " in 1886. From that time the stelar hypo- 

 thesis has replaced all other schemes of vascular anatomy ; 

 the advance then made on all previous generalisations has 

 been shown by the new impulse given to research and the 

 comparative simplicity introduced into text-book anatomy, 

 though equal simplicity cannot be claimed for the technical 



