Feb. I, 1877] 



NATURE 



'99 



easily get their bearings by feeling the direction of the 

 leaves." But the first occasion on which it was brought 

 under the notice of scientific men appears to have been 

 in communications addressed to the National Institute 

 at Washington in August, 1842, and January, 1843, by 

 General Alvord. The accuracy of his statement having 

 been questioned, the General presented another communi- 

 ration at the second meeting of the American Association 

 for the Advancement of Science, held at Cambridge, Mass., 

 in August, 1849, i'^ which he confirms his own observa- 

 tions by those of other officers, Sll agreeing in the con- 

 clusion that the radical leaves of the plant really present 

 their edges north and south, while their faces are turned 

 east and west, the leaves on the developed stems of the 

 flowering plant, however, taking rather an intermediate 

 position between their normal or symmetrical arrange- 

 ment and their peculiar meridional position. For the 

 lollowing particulars respecting the phenomena exhibited 

 by the compass-plant we are indebted mainly to a 

 paper by Mr. W. F. Whitney, read before the Harvard 

 Natural History Society and printed in the American 



The Compas!^-p'ant {Silphiwit laciniatwn\ 



Naturalist for March, 1 871, and to some subsequent notes 

 by Prof. Asa Gray and Mr. Thos. Meehan. 



The cause of the ordinary position of the leaves of 

 most plants, one surface being directed towards the sky 

 and the other towards the earth, is generally believed to 

 be a difference in the sensitiveness to light of the two 

 surfaces, the epidermal tissue of the upper being gene- 

 rally denser and less pervious to light than that of the 

 under surface. It is possible also that something may 

 be due to the fact that the under surface of the leaf is 

 almost always more copiously furnished with stomates or 

 "breathing- pores," as they are often inaccurately termed, 

 minute orifices, which serve to promote a diffusion of 

 gases between the external air and the intercellular cavi- 

 ties within the tissue, and especially an abundant exhala- 

 tion of aqueous vapour. A microscopical examination of 

 the leaves of the compass-plant shows that the structure 

 of the epidermal nssue of the two surfaces is similar, 

 and also that the number of stomates in each corre- 

 sponds, affording in this respect a contrast to other allied 

 species of the genus" Silphium, which do not exhibit 

 the phenomenon of polarity, and in whose leaves the 



stomates were found to be from two to three times as 

 numerous on the under as on the upper surface. If, 

 therefore, the object to be gained is an equal sensi- 

 tiveness to light, it is obvious that the two surfaces will 

 receive an equal mean amount of light during the twenty- 

 four hours, it they face the east and the west, rather than 

 if they face the north and the south, or the earth and the 

 sky. An attem})t has also been made to explain the 

 phenomenon of polarity by currents of electricity induced 

 by the peculiar chemical composition of the substances 

 secreted by the stem and the leaves, but not with much 

 success. 



In a recent communication to the Academy of Natural 

 Sciences at Philadelphia, Mr. Meehan says that those 

 who affirm that the leaves are directed to the north, 

 and those who say that there is no such tendency, are 

 both right. He watched a plant in his own garden, and 

 observed the unmistakable northern tendency in the 

 leaves when they first came up, and until they were large 

 and heavy, when winds and rain bore them in different 

 directions, and they evidently had not the power of regain- 

 ing the points lest. It would appear, therefore, to depend 

 on the season when the observation is made whether the 

 leaves are seen to bear northwards or not. 



The same observer records also the following facts 

 with regard to the flowers of the compass-plant. The 

 expansion of the ray-florets in August was observed to 

 begin at daybreak, 4 A.M. ; forty minutes afterwards the 

 florets of the disc began to open, and the whole of the 

 corollas were expanded in about three-quarters of an 

 hour, after which there was no further growth in the 

 corollas. The stamens and pistils were gradually pro- 

 truded beyond the corolla, and the lengthening of the 

 stamens ceased at 6 A.M. After 6.20 no further growth 

 was noticed in the flowers. Later on they were visited 

 by insects, causing the detachment of the florets of the 

 aisc, and the scattering of the pollen on to the ray-florets, 

 which were thus fertilised. There appear, in fact, to be 

 three phases of growth, with a slight rest between each, 

 the pistil taking the most time, then the stamens, and the 

 corolla the least ; but the whole growth of the day is 

 included within two hours. 



The geographical range of the plant is stated to be 

 from Texas on the south to Iowa on the north, and from 

 Southern Michigan on the east to 300 or 400 miles west 

 of Missouri and Arkansas. Its chief habitat is rich 

 prairie land. Our illustration is taken partly from the 

 plate in Jacquin's " Eclogje," the only good drawing of 

 the plant published, assisted by comparison with dried 

 specimens in the Kew Herbarium. A. W. B. 



EXPERIMENTS WITH THE RADIOMETERS 

 II. 



HAVING ascertained that the action of the radio- 

 meter was due to the internal movement of the 

 molecules of the residual gas, it became important to 

 obtain as much information as possible respecting the 

 physical properties of this residue. 



In the apparatus constructed for this purpose a vertical 

 plate is suspended by a glass fibre, which it twists in 

 opposite directions alternately, instead of continuously 

 rotating in one direction, as m the ordinary radiometer. 

 Attached to this apparatus there is \—a, a sprengel pump ; 

 b^ an arrangement for producing a chemical vacuum ; <r, 

 a lamp with scale, on which to observe the luminous index 

 reflected from a mirror ; d, a standard candle at a fixed 

 distance ; and <?, a small vacuum tube, with the internal 

 ends of the platinum wires close together. I can there- 

 fore take observations of — 



I. The logarithmic decrement of the arc of oscillation 

 when under no influence of radiation. 



' Continued from p. 227. 



