NYCTITROPISM 



501 



The reason why we associate nyctitropism with haptotropism is that 

 the mechanics of nyctitropic movement may, in many cases, be compared 

 with that exhibited by tendrils ; the hkeness between these two sets of phe- 

 nomena is especially evident if we compare the thermonastic curvatures of 

 tendrils with the nyctitropic movements which we find manifested by many 

 flowers. The thermonastic curvatures of tendrils arise, as Fitting (1903) has 

 established, exactly in the same way as do haptotropic curvatures. On cooling 

 or heating a growth acceleration makes its appearance, which is most vigorous 

 on the upper side near the periphery, but which spreads to the middle zone also, 

 and fades away in the neighbourhood of the contracting under or concave side. 

 Some time afterwards, when the temperature has again become constant, the 

 tendril once more straightens itself as a result of an inverse growth process. 

 Let us now compare this with what we see in a spring flower, such as a tulip 

 or a crocus. When the temperature has risen sufficiently, vigorous growth takes 

 place on the upper side of the perianth ; this becomes convex, and the individual 

 leaves bend more or less outwards, in accordance with the rise in temperature. 

 Visible curvatures may be distinguished even after the application of feeble 

 stimuli ; in the crocus, for example, an elevation of temperature of one half a 

 degree is, according to Pfeffer (1873), sufficient to cause a movement. In the 

 tulip the greatest curvature takes place in the basal part of the perianth leaves, 

 and we may convince ourselves of this fact by carefully measuring fixed distances 

 of appropriate length on opposite sides of the perianth leaf. In the following 

 table, measurements of this kind are recorded which aim at showing the effect 

 of increase of temperature on the rate of opening of two flowers. The numbers 

 represent percentage increments per hour. The flowers were kept at a tempera- 

 ture of 11° C. from 5.30 p.m. one evening until 12.40 p.m. the next day, when 

 they were transferred to a temperature of 18° C. (Jost, 1898). 



If we consider the first flower, we find that at a constant temperature of 

 11° C. an extremely slight increase takes place in the course of several hours ; 

 but in the first hour after the higher temperature had made itself felt the 

 inside grows rapidly, the outside not at all. In the second hour, the relations 

 are entirely reversed ; the outside grows rapidly, the inside scarcely at all. The 

 second flower behaves in exactly the same manner. The curvature of the 

 perianth leaf naturally goes hand-in-hand with this distribution of growth, that 

 is to say, in the first hour the flower opens vigorously, and during the second 

 hour as vigorously closes. This phenomenon corresponds in all respects to 

 that exhibited by tendrils, for the stimulus results not in a movement in one 

 direction only, but in a backward and forward oscillation. As in the case of 

 tendrils, the time which is necessary for the carrying out of an oscillation varies 

 greatly in different flowers. The backward movement follows very rapidly 

 in the case of Tulipa, but it is not so in all cases ; in Crocus, two or three hours 

 elapse, and in other flowers an even longer period, before the backward move- 

 ment begins. It can scarcely be doubted that we have here to do with a case 

 of autotropism, and we may imagine that it is due, as is the curvature itself, to 

 a growth acceleration in the median zone also. It is possible that between those 

 two periods of more vigorous growth there intervenes a time when growth 



