148 PLANT PHYSIOLOGY 



on the upper side has returned to the normal state ; the pause which FITTING 

 (1904) observed to occur in tendrils between the two periods of accelerated 

 growth is absent. The fact that in the crocus (Fig. 157 a) a pause occurs 

 just as in tendrils shows that no special importance attaches to this point. 

 The backward movement takes place here very much later than in the tulip, 

 often, in fact, about three hours. 



Fig. 1 57 a. Growth in thermonastically stimulated flowers (after WIEDERSHEIM, 

 1904). Percentage increase in growth of the upper (thick line), under (thin line), and 

 median (dotted line) regions. A, tulip, after transference from 7-5 to 26 C. ; B, crocus, 

 after transference from 9-3 to 20-8 C. The abscissa shows time in hours from the com- 

 mencement of stimulation ; the ordinates, growth increments. 



The similarity between thermonastic floral movements and tendril move- 

 ments is much increased if the behaviour of the two types of organ be observed 

 under conditions which do not permit of curvature taking place. WIEDER- 

 SHEIM (1904) was able to establish a pause between two successive periods of 

 growth acceleration in floral leaves which were fixed, as FITTING had found in 

 tendrils. From this it must be concluded that a reverse movement takes 

 place in flowers not only after an accomplished curvature but also after the 

 effort to curve. A further analogy with tendril movements shows itself in the 

 fact that in floral movements also every time the temperature is raised a new 

 stimulus is administered, both at the period of incurving and of recurving 

 also ; the plants do not become accustomed to the stimulus, or do so only 

 quite gradually, and it is possible to induce opening movements with appro- 

 priate elevations of temperature for many hours (JosT, 1898). 



In spite of all these analogies, one must not forget the differences which 

 exist between tendril and floral-leaf movements. These stand out prominently 

 when we consider the effect of cooling on the perianth leaves. Cooling acts 

 precisely in the reverse way to heating, i.e. it accelerates growth on the under 

 sides of the leaves and induces closure of the flower. In tendrils, however, 

 even in those which are haptotropically sensitive all round, a rise in tempera- 

 ture induces the same curvature as a fall. Apart from the direction of cur- 

 vature, the stimulus movement in flowers, after cooling, is of the same character 

 as that after heating ; it is the result of more vigorous growth in the middle 

 zone. This is seen from the following measurements made by PFEFFER (1875) 

 of the growth in length of the middle zone of the crocus in percentages per 

 hour : 



At i7-i8C. At 7-7|C. 



4 p.m.-9 a.m. 9 a.m. -12 noon. 

 Crocus No. i 0-64 0-70 



Crocus No. 2 0-67 0-74 



ist hf. hr. 2nd hf. hr. next 3 hrs. 

 4-65 1-87 0-41 



6-21 3-27 0-34 



It is not known whether an opening movement, i.e. an autotropic reverse 

 action, takes place at continued lower constant temperatures, but it is very 

 probable that that is the case. 



Summarizing, we have established that the act of changing the tempera- 

 ture causes the average growth of the perianth leaves to exceed quite markedly 

 the amount attained in the long run when the temperature is constant. A 

 temperature so high or so low that it at last stops growth may indeed, at the 

 moment of application, even induce a growth acceleration (comp. BURGER- 

 STEIN, 1902). This acceleration is partial and occurs indeed on the under or 

 upper side of the perianth, according as the temperature is lowered or raised. 

 When the stimuli are vigorous the acceleration overshoots the mark the new 

 position of equilibrium and it is then compensated by a second growth 

 movement setting in on the opposite side. 



11. 48 and 50, for nyctitropic read nyctinastic 



11. 51-2, delete [As to other . . . 1905).] 



