282 



KNOWLEDGE 



[Decembeb 1, 1898. 



however, are sufficiently bright and well separated to serve 

 for comparison in the discovery of variables. Of these 

 three thousand, one hundred and twenty-five are variable. 

 One hundred and fifty photographs of the cluster have 

 been taken with the thirteen-inch telescope, and already 

 ten thousand measures have been obtained, about half of 

 which have been made by Miss E. F. Leland. 



Although the results are at present provisional, it is not 

 probable that the final results of the discussion will 

 materially alter the conclusions. Of the hundred and six 

 variables in m Centauri whose periods have been deter- 

 mined, ninety-eight have periods less than 24h. The 

 longest period is that of No. 2, 475d., the shortest that of 

 No. 91, 6h. 11m. Three have periods less than 7h. Of 

 the eight having periods of more than 21h., two have 

 periods between one and two days, two between two and 

 three days, one of four days, one of fifteen days, one of 

 one hundred and fifty days, and one of four hundred and 

 seventy-five days. 



The largest range in variation is about five magnitudes, 

 and no star has been included whose light changes do not 

 amount to half a magnitude. 



The light curves of the ninety-eight stars whose periods 

 are less than twenty-four hours may be divided into four 

 classes. The first is well represented by No. 74. The 

 period of this star is 12h. 4-3m., and the range in bright- 

 ness two magnitudes. Probably the change in brightness 

 is continuous. The increase of light is very rapid, 

 occupying not more than one-fifth of the whole period. 

 In some cases, possibly in this star, the light remains 

 constant for a short time at minimum. In most cases, 

 however, the change in brightness seems to be continuous. 

 The simple type shown by No. 74 is more prevalent in 

 this cluster than any other. There are, nevertheless, 

 several stars, as No. 7, where there is a more or less well 

 marked secondary maximum. The period of this star is 

 2d. llh. 51m., and the range in brightness one and a half 

 magnitudes. The light curve is similar to that of well- 

 known short-period variables as S Cephei, and vj Aquilae. 

 Another class may be represented by No. 126, in which 

 the range is less than a magnitude, and the times of 

 increase and decrease are about equal. The period is 

 8h. 12-3m. No. 24 may perhaps be referred to as a fourth 

 type. The range is about seven-tenths of a magnitude, 

 and the period is llh. .5'7m. Apparently about sixty-five 

 per cent, of the whole period is occupied by the increase 

 of the light. This very slow rate of increase is especially 

 striking from the fact that in many cases in this cluster 

 the increase is extremely rapid, probably not more than 

 ten per cent, of the whole period. In one case, No. 45, 

 having a period of 14h. 8m., the rise from minimum to 

 maximum, a change of two magnitudes, takes place in 

 about one hour, and in certain cases, chiefly owing to the 

 necessary duration of a photographic exposure, there is no 

 proof, at present, that the rise is not much more rapid. 



The marked regularity in the period of these stars is 

 worthy of attention. Several have been studied during 

 more than a thousand, and one during more than five 

 thousand periods, without irregularities manifesting them- 

 selves. 



A few words may be added in regard to the kind of 

 clusters in which variables have been found. Up to the 

 present time they have not been found in any except dense 

 globular clusters, of which Messier 3, Messier 5, and the 

 great cluster in Hercules may be taken as examples. The 

 number of such clusters within the reach of ordinary 

 instruments is not great. Of the clusters given in the 

 table, N. G. C. 104, 362, 5139, 52'^2, 5904, 6093, 6205, 

 6266, 6620, 7078, and 7089, may be described as highly 



condensed; 1904. 5986, 6397, 6656, 6723, 6752, 6809, 

 and 7099, as moderately condensed ; and 3293 and 4755, 

 as open clusters. 869 and 884, the clusters in the sword- 

 handle of Perseus, are little more than regions relatively 

 rich in stars. 



The first group, of eleven highly condensed clusters, 

 having a total of eleven thousand nine hundred and eighty 

 stars, has four hundred and sixty-two variables, or one in 

 twenty-six. The second group, of eight moderately con- 

 densed clusters, has forty-six variables among four thousand 

 seven hundred and forty-one stars, one in one hundred and 

 three. The two open clusters furnish no variables, and 

 the region of three square degrees around N. (i. C. 869 

 and 884 only one. 



Thus far the only regions which are found to be especially 

 rich in variable stars are condensed clusters, but even here 

 only in relatively few cases. These dense clusters are 

 commonly called globular, and many of them are such. 

 In some cases, however, as lu Centauri, the form is some- 

 what elliptical. 



N. G. C. 6266 is the most striking example of a highly 

 condensed cluster which is irregular in form. This irregu- 

 larity is intensified in the distribution of the variables. 

 The cluster is much compressed on the south side. For a 

 distance of one minute from the centre the distribution of 

 the stars seems to be about equal, but if a line be drawn east 

 and west through the centre, and the stars within one 

 minute of this line are omitted, there are two hundred and 

 fourteen stars south, and three hundred and fifty-four stars 

 north, within four minutes of the central line. In this clus- 

 ter are twenty-six variables, of which nineteen are north of 

 the central Hne, and seven south. Excluding those within 

 seventy inches, there are fifteen north, and only one south. 



BOTANICAL STUDIES. 



VII.— ABIES. 



By A. Vauouan Jennings, f.l.s., f.g.s. 



THE results of our study of the mode of reproduc- 

 tion and life-history of Selaginella (Knowledge, 

 November, 1898) may be shortly recapitulated as 

 follows :— The plant bears two kinds of sporangia 

 (mairosporanijia and microsporanf/ia) instead of 

 only one, as in the Ferns, and these contain two kinds of 

 spores (macrospores and microspores) ; the large spores 

 seem to give rise to the new plant but only by means of an 

 excrescence of tissue which proves to be the representative 

 of the Prothallus or Oophyte of the Fern, here reduced, 

 colourless, rootless and dependent on the spore ; that this 

 protuberance contains true, but rudimentary, Archegonia, 

 the egg-cells of which are fertilized by spermatozoids de- 

 veloped from the microspores, which also show a tendency 

 to form a still more rudimentary prothallus. 



In passing on to the Mountain Pine, the Ahies excelsa 

 which clothes the slopes of our European mountains, we 

 are crossing a great gap in the series of plant types : and 

 yet, while we do not know the forms which bridge the 

 gulf, once more the use of patience and a microscope has 

 shown how these widely dilierent types can be compared, 

 co-related, and brought into line with the other plants we 

 have been examining. 



In botanical classification the gap wa are crossing is 

 that between the Cnjptofiamia and Phanerogamia. All the 

 types we have so far studied belong to the former division. 

 The name suggested itself to the early naturalists as one 

 to express the fact that the lower plants have no con- 

 spicuous " flowers " such as characterize the majority of 

 the higher forms. 



