June 1. 1892.] 



KNOWLEDGE 



111 



have been a few inches or a foot in diameter. Thus the 

 energy lost hy the meteor becomes distributed as heat 

 through a region considerably larger than that originally 

 disturbed by the passage of the body. 



If such a disturbance took place on a stellar scale, we 

 should expect to find the matter of the star mixed with 

 nebulous matter driven into a state of intense gaseous 

 incandescence in front of the moving star, and the material 

 left in its wake probably at first ui a state of brilliant 

 gaseous incandescence, but soon condensing iato incandes- 

 cent particles, which would give out a continuous spectrum 

 and form a sort of elongated photosphere, the light of 

 which would be channelled by the absorption due to the 

 cool surrounding nebulous matter. We should thus have 

 a bright-line spectrum in which the position of the lines 

 corresponds with the velocity of the moving star, and an 

 absorption spectrum corresponding with the velocity of the 

 absorbing nebulous matter. 



The lines which we find as bright in the spectrum of the 

 Nova correspond with lines which are the last to glow in 

 the solar chromosphere, as the matter shot up in the 

 prominences cools. ' They, therefore, correspond with the 

 lines which would be the first to glow on being heated. 



It is evident that the nebulous matter must be very 

 sparsely distributed, for the velocity of the star does not 

 seem to have appreciably altered in the three weeks, 

 during its passage through the densest part of the nebula. 

 In this period, it must have passed across a region more 

 than equal to the diameter of the orbit of -Jupiter. During 

 the whole period in which the star, according to Prof. 

 Pickering's observations, remained above the eleventh 

 magnitude, it must have passed across a nebulous region 

 of greater diameter than the orbit of Uranus. If the matter 

 of the solar system were distributed uniformly through a 

 sphere of the same diameter as the orbit of Uranus, we 

 can calculate the mass of the matter which would be 

 cut through by the passage of a body as large as the earth 

 diametrically across such a nebula ; and though no doubt 

 a much larger mass of the nebula would be displaced and 

 heated, it seems probable that a body of the same mass as 

 the earth would not lose an appreciable portion of its 

 velocity in giving an equal velocity to the quantity of 

 matter in the first instance disturbed. 



A very significant fact as to the distribution of these 

 temporary stars is that they have all appeared in and 

 about the region of the Milky Way, which is a region of 

 extensive nebulie. One of the most recently observed of 

 these temporary stars appeared actually near to the 

 centre of the great Andromeda nebula, and its spectrum 

 did not appreciably dift'er from the spectrum of tbe nebula. 

 If it exhibited bright lines, they were, like the bright lines 

 of the Andromeda nebula spectrum, exceedingly faint. 



THE FLOWER OF MAHOMET. 



By the Eev. Alex. S. Wilson, M.A., B.Sc. 



THE Prophet-plant (Arnehia echioidcs) is a native of 

 Persia and Arabia, but has been introduced and 

 grows freely in gardens in this country. Its chief 

 interest lies in its variable flowers, which may 

 fairly rank with those of the changeable Hibiscus 

 and other 



" Plants cliviue and strange 

 That even- hour their blossoms change." 



* The vertical stonus continually taking place upon the sun 

 preclude the idea that the upper chromosphere is a region only occu- 

 pied by hydrogen and 1474: stuff. The gaseous matter of the sun 

 must be completely mixed by diffusion as well as churned by solar 

 storms. But in the outer and cooler regions certain elements 

 continue to glow when others have become non-luminous. 



The plant is about two feet in height, and somewhat 

 resembles a cowslip or an auricula. It belongs to tbe 

 natural order Boraginacese, and is nearly allied to the 

 lungwort, viper's-bugloss, borage and forget-me-not, all of 

 which exhibit colour-changes more or less distinct. The 

 various sjiecies of Myosotis or forget-me-not are also called 

 scorpion grasses, from the upper flower-bearing portion of 

 the stem being curled on itself like a watch-spring. The 

 cluster of flowers, forming tbe inflorescence of .\rnebia, 

 develops in the same scorpioid fashion. There is a double 

 row of flower-buds on tbe curled stalk, and as this 

 gradually unwinds pair after pair of the flowers expand in 

 succession. In shape and colour the individual flowers 

 are not unlike those of the primrose, though rather 

 smaller. When a flower first opeus five conspicuous jet 

 black spots are seen upon the yellow rim of the salver- 

 shaped corolla. If the flower be examined the following 

 day, we are surprised to discover that the black spots have 

 vanished as if by magic. The yellow of the corolla is 

 also much paler, and a little later on presents quite a 

 bleached or silvery appearance, the petals becoming almost 

 white. No sooner have tbe spots disappeared from the 

 first pair of flowers than a second pair expand, and display 

 their sable marks in bold relief upon tbe yellow enamel 

 of their petals. From this time onwards the inflorescence 

 comprises both kinds of flower, tbose but newly opened 

 having the five conspicuous spots, and older ones on which 

 no spots are visible. From these dark spots — the so-called 

 finger-marks of Mahomet, Arnehia has received its name — 

 the Prophet-plant. Its flowers seem bewitched, the 

 change is so pronounced and obvious ; a day or two after 

 unfolding they dift'er so much from the newly-opened ones 

 beside them, that were they growing on separate plants, 

 we should at once set them down as belonging to another 

 species. 



This change of colour gives rise to another interesting 

 peculiarity. If Arnebia be examined by daylight, and 

 again in the dim twilight, the observer is struck by a 

 remarkable circumstance. In broad daylight, the golden 

 spotted flowers at once arrest the eye, while their paler 

 companions are hardly observed. The inflorescence owes 

 by far tbe greater part of its display to the younger flowers. 

 In the dusk this is entirely reversed ; the eonspicuousness 

 of the inflorescence now depends on the paler flowers, and 

 the others are so obscured that a second glance is needed 

 before they can be discerned. The relative brilliancy of 

 the two sets of flowers can also be tested by gradually 

 retiring from the plant, keeping the eyes still fixed on the 

 blossoms. At dusk the young flowers are lost sight of 

 much sooner than the others : by day the older ones first 

 disappear in the distance. This peculiar transformation 

 imparts to the inflorescence of Arnebia a faint similitude 

 of the pillar of cloud by day and of fire by night — that 

 celestial manifestation of sacred story so closely associated 

 with the native region of this desert flower. 



Here, then, we have one of tbose phenomena which, for 

 the naturalist, possess all the fascination of a mystery. 

 What can be the explanation of this remarkable change of 

 colour, and what advantage does the flower derive from 

 the sudden disappearance of its spots and the blanching of 

 its petals ? 



With the reader's permission, we shall now proceed to 

 show why Nature has bestowed on Arnebia what she has 

 denied to the leopard— the power of changing its spots. 

 Before we can say why any flower should change its colour, 

 we must first know why a flower is coloured at all, and 

 why all flowers are not coloured alike. Almost all tbe 

 peculiarities of flowers can be explained as having 

 reference to the visits of insects. The honey is secreted 



