70 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and closing of flowers and inflorescences ; Plants with ephemeral flowers ; 
Plants with agamotropic flowers ; Nyctitropic movements of leaves ; 
Paraheliotropic movements of leaves ; Irritability of leaves ; Irritability 
of stamens ; Xerochastic curvatures. 
Propagation of Heliotropic Irritability.* — Herr W. Rothert has 
investigated this subject, with a view of deciding between the conflicting 
theories of Darwin and Wiesner. The objects specially observed were 
cotyledons of Avena sativa and PJialaris canariensis , which display re- 
markable heliotropic curvatures, seedlings of Panicum sanguinale and 
miliaceum and Setaria viridis, young plants of Brassica Napus , Tropseolum 
minus , &c. ; in all cases the results were very similar. 
The author finds the capacity for the propagation of heliotropic 
irritation to be widely distributed, though the intensity varies, and in 
many cases it is difficult to detect. In heliotropic seedlings it is very 
usual, though not universal, for the direct heliotropic sensibility — i. e. 
the sensitiveness of the protoplasm to illumination from one side — to 
vary in the different parts of an organ, and the greater degree of sensi- 
tiveness is limited to a comparatively small apical region ; but direct 
heliotropic sensitiveness is never confined entirely to the apex. When 
this sensitiveness is not uniform, the variation is one of the factors in 
determining heliotropic curvature. A distinction must be drawn between 
direct and indirect heliotropic sensitiveness ; the two together make up 
the entire heliotropic sensitiveness of an organ or part of an organ. 
Growth and heliotropic sensitiveness are entirely independent of one 
another ; not only can growth take place without this sensitiveness, but 
there are organs, like the cotyledons of PaniceaB and the internodes of 
Galium , which are heliotropically sensitive after their growth has com- 
pletely ceased. The power of heliotropic curvature of an organ is, 
ceteris paribus, a function of its intensity of growth and of its entire 
heliotropic sensitiveness ; it disappears when either of these functions 
is reduced to zero ; but there may be organs, like the hypocotyl of the 
Paniceee, which curve heliotropically although they have no direct 
heliotropic sensitiveness. 
Experiments on the removal of the head from growing seedlings 
showed that the head acts in two different ways : — in diminishing the 
intensity of growth, and in completely arresting heliotropic and geo- 
tropic sensitiveness. But both these results are only temporary ; after 
a time the rapidity of growth and both kinds of sensitiveness again 
increase ; and, after about twenty-four hours, the normal condition is 
again attained. 
Artificial Production of Rhythm in Plants.! —Prof. F. Darwin 
and Miss D. F. M. Pertz find that, by the use of an intermittent klinostat, 
they can produce a rhythmic movement, i. e. a regular succession of 
nutations in different directions, in young growing plants (valerian, 
dandelion, canary-grass), due to the action of opposite and alternate 
stimuli of a geotropic and heliotropic character. The period of each 
rhythm was, in all cases, almost exactly half an hour. The rhythm 
continues after the conditions which have built it up have ceased to act ; 
* Ber. Deutsch. Bot. Gesell., x. (1892) pp. 374-90. 
f Ann. Bot., vi. (1892) pp. 245-64 (6 figs.). 
