NYCTITROPISM 503 



causes the closure of the capitulum, as cooling does in the flowers of Crocus^ 

 and illumination causes opening, as heating does in the case of Crocus. In 

 nature, as a rule, increased light is accompanied by an elevation and decreased 

 light by a reduction in temperature. Observations as yet available show that 

 curvature either in the lower tubular or in the upper flattened portions of the 

 corolla is effected by the same mechanical means as are thermonastic curvatures. 

 There are various difficulties, due partly to the nature of the movements them- 

 selves and partly to the fact that insufficient measurements have been carried 

 out in this direction, which render it impossible for us to enter into greater 

 detail on these questions at the present moment. 



Nyctitropic growth movements occur in the foliage-leaves of a very large 

 number of plants of very distinct families, e.g. Alsineae, Compositae, Solanaceae, 

 Balsamineae, &c. (Batalin, 1873). As a rule, simple leaves take up a more 

 or less horizontal position by day, while in the evening the laminae assume a 

 vertical position, due either to a curvature in the petiole or at the base of the 

 lamina. Thus the leaves either droop at night time, as in Impatiens, Polygonum 

 convolvulus, Sida napaea, &c., or they stand erect, so as to press themselves 

 against the bud, as in Chenopodium, Brassica, Polygonum aviculare, Stellaria, 

 Linum, &c. That the factor concerned in these movements is an alteration in 

 light may easily be proved in a large number of cases, for placing the leaves of 

 Impatiens in darkness at midday induces a very marked drooping. More exact 

 research is certainly required as to the sensitivity of such leaves for changes in 

 temperature. 



Certain conclusions as to the mechanics of curvature may be drawn from 

 Pfeffer's (1875) measurements, although a more exhaustive investigation of 

 these is urgently needed, especially after Fitting's work on tendrils. The 

 following data are taken from Pfeffer's observations : — 



Impatiens noli-me-tangere. 

 Length of a section originally loo units long. 



Upper side. Under side. 

 9.30 a.m. Light loo loo 



12.30 p.m. I p^^j^ 100 100 



1.30 p.m. \ 105 99.4 



3-30 p.m. Light 105 I02-8 



Although no growth could be observed in the leaf when uniformly illuminated 

 for 3 hours before midday, when the leaf was suddenly darkened the upper side 

 increased 5 per cent, in the course of one hour and the under side decreased \ per 

 cent. ; in the course of 2 hours, after the leaves had been again exposed to light, 

 the under side increased almost 3 per cent., while the upper side remained 

 stationary. The reversion of the curvature, however, which is associated with 

 this more vigorous growth of the under side may also take place without any 

 illumination. It would thus appear as if in this case an oscillation to and fro 

 might be brought about by mere darkening, each movement being accompanied 

 by considerable growth acceleration. The close analogy with tendrils is in no 

 sense supported by this result, since the movements of the leaf of Impatiens are, 

 as we have seen, of a more complicated character. We shall take an opportunity 

 later on of returning to this subject. These same complications render it im- 

 possible for us as yet to answer the question whether the influence of illumination 

 is analogous to that of darkening or not. Comparing the behaviour of flowers 

 when the temperature is raised, we are led to expect a temporary increase in 

 growth when light is intensified ; the evidence in favour of this view is, however, 

 by no means above criticism. [Evidence in favour of this conception has now 

 been adduced by Wiedersheim (1904).] 



Nyctitropic movements in the foliage-leaves hitherto mentioned, are 



