18 



KNOWLEDGE. 



January, 1913. 



Jupiter, + m -57; Saturn, + l m -23. The Sun being of 

 Spectral Class G, a difference of magnitude of m -98 would 

 be expected, and this would be greater for the planets, as 

 reflexion would tend to make the light redder. The deduced 

 photographic albedoes are: — Venus, 0-66; Mars, 0-15; 

 Jupiter, 0-80; Saturn, 0-53. 



The very high value for Jupiter is noteworthy, and suggests 

 that a little of his light may be inherent, especially as some 

 portions of his disc are obviously dusky. 



SATURN. — This planet has been in opposition in 

 November, and now that it is getting into considerable North 

 Declination it is attracting more attention from English 

 observers, especially as Mars and Jupiter are invisible. It is 

 most desirable that the disc should be searched for features 

 sufficiently distinct to determine the rotation period. This 

 differs very widely in different latitudes, from ten and a quarter 

 hours at the equator to 10 h 38™ in the temperate zones; but it 

 is most desirable that these values should be checked by other 

 spots, and that it should be found whether the transition is 

 sudden, or whether intermediate values exist. Spots suffici- 

 ently well marked to be easily identified on their return are 

 not very common, and whenever one is seen it should be 

 utilised to the full, and notice telegraphed to the possessors of 

 large instruments. 



Mr. P. Hepburn, the Director of the Saturn Section of the 

 B.A.A., gave some interesting notes in November, on recent 

 observations. A curious feature, confirmed by several 

 observers, was a dark shading on the ring along the planet's 

 limb, on the side opposite to the true shadow. This is a very 

 perplexing observation, and I have not yet seen any plausible 

 explanation. Mr. Hepburn also points out that careful obser- 

 vations should be made as to where the southern portion of 

 the planet's limb lies on the ring, relatively to its southern 

 edge and the Cassini Division. In this regard it should be 

 remembered that the dimensions of the ring used in The 

 Nautical Almanac do not include the latest measures, and 

 that their value of the polar semi-diameter of the planet is 

 faulty ; what they give is the value of the actual polar radius 

 as it would look if seen unforeshortened ; but what we 

 observe is a radius of the planet which is now twenty -four and 

 a half degrees distant from the pole, and therefore decidedly 

 longer. 



LIMITING DISTANCE FOR HYPOTHETICAL 

 SATELLITES OF NEPTUNE.— Mr. C. T. Whitmell 

 recently gave me an interesting problem, viz., to find the 

 greatest distance at which a satellite of Neptune need be 

 searched for. Dr. G. W. Hill in his researches on the 

 Lunar theory, gave the elements of a satellite which had 

 1-7 lunations in the planet's year. The shape of the orbit 

 is curious ; its distance from the planet at Syzygies is only 

 one-third of that at quadrature, and when the motion is 

 referred to the line joining sun and planet (considered as 

 fixed), there are cusps at the quadrature points. In the case of 

 Neptune I find that such a satellite would be distant 0-30 astro- 

 nomical units from Neptune in syzygies and • 87 in quadrature. 

 If the latter took place in opposition the satellite would appear 

 L7 degrees distant from Neptune. As there is little chance 

 of an actual satellite being so distant we may take one degree 

 from the planet as the limit of useful search. A good deal of 

 search has already been carried on by photography, but there 

 is plenty of room for further efforts. In the case of very 

 close satellites visual search would be more promising. All 

 the probabilities point to Neptune having other satellites, but 

 at such a distance only very large ones can be seen. Such 

 tiny bodies as Phoebe, the outer satellites of Jupiter, or even 

 (probably) the four satellites of Uranus would be beyond our 

 reach. 



PROFESSOR FOWLER'S DISCOVERY OF A NEW 

 SERIES OF LINES IN THE SPECTRUM OF 

 HYDROGEN. — Four sets of lines arranged so as to form 

 regular rhythmical series were already known in the spectrum 

 of hydrogen. A few stellar spectra showed evidence of a fifth 

 series related to the fourth in a simple manner that suggested 



that it belonged to the same element. The lines were, 

 however, much fainter than the others, and had not been 

 detected in the spectrum of terrestrial hydrogen. This Professor 

 Fowler has now succeeded in doing, taking the spectrum of 

 the gas in a low-pressure tube, mixed with a little helium. 

 He has not yet succeeded in getting the lines in the absence 

 of the helium, but their exact accord with the formula for the 

 fourth series, when one of the constants is halved, leaves 

 practically no doubt that they belong to hydrogen. His 

 paper was read at the December meeting of the Royal 

 Astronomical Society. 



BOTANY 



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



THE GENUS LEITNERIA.— In Engler's system of 

 classification the genus Leitneria, constituting the order 

 Leitneriaceae, is placed in a separate cohort (Leitneriales) 

 near the base of the Lower Dicotyledons (Archichlamydeae) 

 on account of its catkin-like inflorescence and simple flower 

 structure. The two species of Leitneria, confined to North 

 America, are shrubs with spikes of dioecious flowers ; the 

 male flower has from three to twelve stamens and no perianth, 

 while the female has a scaly perianth and one carpel with a 

 long style, the unilocular ovary containing a single ovule. 

 Pfeiffer (Bot. Gaz., LIIL, 3) has made a thorough examination 

 of the development of the flowers of this isolated genus, and 

 finds that the ovule contains a single archesporial cell, differing 

 from most of the Amentiferae or catkin -bearing trees in this 

 respect, though the same condition occurs in Bctula alba, 

 Alnus, and some species of Salix — hence a multicellular 

 archesporium can hardly be considered a group character. 

 The young embryo is pear-shaped, and has a massive 

 suspensor ; there is no fixed sequence of cell-divisions in its 

 development. The most striking features of Leitneria, how- 

 ever, are the resemblances which it shows to Gymnosperms; 

 but such resemblances are also found commonly among the 

 Amentiferae. The author's conclusion is that while the 

 morphology of Leitneria is not such as would make it possible 

 to place it definitely in any of the families of Archichlamydeae, 

 it agrees with the Amentiferae in suggesting the possibility of 

 the derivation of the lower Dicotyledons from Gymnospermic 

 forms with compound inflorescences. 



DEVELOPMENT OF SALVINIA.— Few groups of plants 

 have been so much worked at as that containing the four 

 " water-ferns " or Hydropterideae, though it is now generally 

 recognised that the two families formerly classed together 

 under this name ought to be separated somewhat widely, the 

 Salviniaceae (Salvinia and Azolla) having probably arisen 

 from a homosporous family like the filmy ferns (Hymeno- 

 phyllaceae), while the Marsiliaceae (Marsilia and Pilularia) 

 are probably connected with the Schizaeaceae. The most 

 striking character of these two families is, of course, their 

 heterospory — they are the only ferns with two kinds of spores 

 — but they present many other interesting features. In a 

 recent paper, Zawidski (Beihefte zum Bot. Centralbl., Band 

 28, Abt. 1, 1912) gives an elaborate account of the development 

 of the vegetative organs and sporangial receptacles (sori) of 

 Salvinia nutans, clearing up various points and giving a very 

 complete picture of the morphology of this type. The growth 

 of the shoot takes place with great regularity ; the stem grows 

 by a two-sided apical cell, and soon shows differentiation into 

 nodes and internodes, each node showing four peripheral cells 

 which form the initials for the two floating leaves, the sub- 

 merged leaf, and the branch. All these organs grow by a two- 

 sided apical cell, and this is also the case with the sori, each 

 sorus being a metamorphosed segment of a submerged leaf. 

 The oldest sorus produces the large spores (megaspores), the 

 others produce the small spores (microspores). 



MASSULAE OF AZOLLA.— In connection with the 

 preceding note, mention may be made of a recent paper 

 by Hannig {Flora, Band 102) on the development of the 

 massulae of Azolla. This plant is, like Salvinia, a free- 



