1291 
After 2 hours the curvatures amounted to: 
User C | 4 mg 
| 
| 
| 
2.5 | 1.5 1.5 
The influence of the longitudinal component is therefore once more 
evident. 
The term longitudinal component of gravity is of course only a 
phrase. No way of explaining it physiologically has so far beer found. 
The phototropic curvatures of the coleoptiles of Avena when 
illuminated at different angles, showed a very marked deviation from 
the expected sine relation. As Arisz (l.c. 1914) has justly argued, 
the paraboloid shape of the apex must be a very important factor 
in this connection. SN 
In geotropie reactions another factor must also be taken into 
account, namely, the polarisation of separate cells. 
It is generally assumed that a difference exists in the sensitiveness 
to pressure of the protoplasm lining the inner and the outer walls 
of the cells. The idea that there may be a similar difference of 
sensitiveness between the apical and basal part of each cell, may 
therefore not be summarily rejected. In this way the longitudinal! 
component can also be explained. In the rotating apices of climbing 
plants where I could establish its influence on growth as well as on 
the nature of geotropie curvature, this is probably the right conception. 
The paraboloid vegetation point of a stem which bents at its end 
like a hook, may here take up any sort of position and hardly 
deserves consideration in connection with gravitational stimuli. 
Utrecht, March 1915. Botanical Laboratory. 
Astronomy. — “On the mean radius of the earth, the intensity 
of gravity, and the moon’s parallax. By Prof. W. De Sirrer. 
1. Newcoms has more than once‘) pointed out that the mean 
radius of the earth is more appropriate for use as a standard of 
reference, than the equatorial radius, which is always used in 
astronomical practice. The mean radius in fact, which — if we 
neglect quantities of the second order in the compression — is also 
the mean radius of curvature, is more nearly the quantity actually 
1) Researches on the motion of the moon, second paper, page 41 
Tables of the sun, page 12, footnote. 
