GROWTH AND MOVEMENT 443 



from which the stimulus acts but the structure of :he organ predetermines 

 the plane of the movement. 



Light and temperature. Examples of paratonic nastk curvature are 

 seen when light and temperature act as stimuli upon foliage and flower 

 leaves, and less plainly in tendrils. Temperature changes are especially 

 effective with the peri|anth leaves of tulip, crocus, snowdrop, colchicum, 

 and other plants whose blossoms appear very late in the autumn or very 

 early in the spring. In the crocus a rise of half a degree suffices to bring 

 about a curvature that opens *he flower; while the tulip can be made 

 to open and close as many as eight times in the course of an hour by 

 raising and lowering the temperature. Tendrils respond to a tempera- 

 ture change, whether a rise or a fall, by curving in one direction only, 

 the upper side being stimulated to accelerated growth. In this they differ 

 from the perianth leaves cited, for in these a rise of temperature tends 

 to accelerate growth on the inner face and thus to open the flower, and 

 a fall to accelerate growth of the outer face and so to close the flower. 

 Very many, perhaps the majority of foliage leaves, show nastic curvatures 

 in response to alterations in temperature and light as long as the petiole 

 is still capable of growing; finally curvature ceases and the fixed light 

 position of maturity is attained. Such bending movements remind one 

 of the photeolic movements executed throughout life by leaves that have 

 motor organs (see p. 451). Among flowers those most strikingly re- 

 sponsive to light are the heads of some Compositae, such as the dande- 

 lion. Here the flowers and the bracts about the flower cluster, the 

 involucre, curve so as to close the head when the light is diminished, as 

 in cloudy days, and to open it in sunshine (Part III, figs. 1193, 1194). 



In countries where the climate is equable it is possible to select plants whose 

 flowers open at particular hours of the day on. account of light and temperature 

 stimuli, and by planting them in a circle to have a sort of floral clock. Naturally 

 it is not very reliable. 



Gravity. Nastic curvatures are also produced in plants in response 

 to gravity, which, however, usually cooperates with or antagonizes the 

 light reactions. In all cases the stimulus at work may be indicated by 

 the prefix. Thus we have photonasty, thermonasty, etc., and still more 

 specifically photepinasty, geohyponasty, etc. 



Mechanism. In all these curvatures the mechanism of response is 

 the same. The growth of the outer or inner surface is accelerated, as 

 can be shown by making equidistant marks upon the two faces and mea- 

 suring the changes, This observation shows, too, that under frequent 



