James Small. 
216 
quoted, except that the pressure difference was 5 cms. gave a 
reading at 11.25 a.in. of 026 seconds per bubble (30 bubbles timed) 
and by 4.20 p.m. on the same day, the time for one bubble was 
more than 300 seconds. This was an extreme case, and in general 
the conditions of the experiment are arranged so that a bubble 
never takes less than one second to form, a very rapid stream, 
besides being difficult to time, being liable to cause one bubble to 
interfere with the next. At the other extreme, readings of hundreds 
of seconds can hardly be regarded as anything but an approximate 
average, for in two or three minutes some stomata may open or 
close to a considerable extent. 
Whenever possible, each reading is taken three times in rapid 
succession, and the mean is regarded as an indication of the 
stomatal aperture, unless this is changing very rapidly as is 
sometimes the case towards evening. 
Imperial College of Science and Technology, 
May, 1915. 
PRELIMINARY OBSERVATIONS ON THE POLLINATION 
MECHANISM OP ARCTOTIS ASPERA LINN. 1 
By James Small, B.Sc. (Lond.), Ph. C., 
Demonstrator in Botany, Armstrong College, Newcastle-on-Tyne. 
[With Six Figures in the Text]. 
VERYONE has heard of the sensitive stamens of Centaurea 
cyanus, but it is not so widely known that other cases of 
irritability occur in the Composite, especially in the Cynareae, 
e.g., Centaurea montana, Centaurea dealbata, Carduus Kerneri, 
Silybum marianum, Gerbera multiflora. There are also other 
examples. In the allied tribe, Arctotideae, is the species under 
consideration, but in this case it is the style which is sensitive. 
The course of events is unusually complex and a description of 
the composition of the capitulum and of its development from the 
bud is the clearest method of explaining the behaviour of the florets. 
1 Since this paper was written, my attention has been called to the fact 
that M. von Minden (“ Reizbare Griffen von zweier Arctotis- Arten,” Flora, Bd. 
88, 1901, p. 238) has described in some detail, though without figures, the 
movements of the style in Arctotis aspera and A. calendulacca. I have been 
unable to see Minden’s paper, but am informed that his observations are 
precisely similar to those here recorded. However, it has seemed worth 
while publishing the present account, especially as no figures appear to have 
been published previously to illustrate the interesting facts observed- 
