502 



TRANSFORMATION OF ENERGY 



is retarded or ceases entirely ; measurements taken at shorter intervals than 

 have as yet been made can alone settle this question. [Measurements of this 

 kind have been made meanwhile by Wiedersheim {1904) with results precisely 

 of the nature one would expect.] The growth acceleration of the middle zone 

 associated with the movements we are discussing is remarkably apparent in 

 the measurements above quoted. 



Hourly percentage of growth in the median zone. 



The acceleration shows itself most clearly if we compare growth in the 

 first and second hours after warming with that exhibited in the fourth hour 

 when exposed to continuous but uniformly higher temperature. 



The effect of cooling may best be studied in the measurements recorded by 

 Pfeffer (1875, p. 125). He subjected crocus flowers, which had been exposed 

 for about twenty-four hours to a temperature of 17° to 18° C, to a temperature 

 of 7° to 7i° C, and observed that the flowers closed. Increased growth set in 

 on the outsides of the perianth leaves, which again affected the middle zone, 

 but which after a short time became very considerably reduced. In support 

 of this statement we give below certain of Pfeffer's measurements of the 

 growth of the middle zone of the crocus in percentages per hour. 



It is unknown whether an opening movement, i. e. an autotropic reverse 

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

 that that will be found to be the case. 



The act of changing from one temperature to another increases the average 

 growth of the perianth leaves much above that exhibited at constant tempera- 

 tures ; it affects the upper and under sides differently, however, and thus induces 

 curvature. In all probability, a curvature may even make its appearance at such 

 temperatures as lie beyond the maxima and minima of growth when they affect 

 the plants constantly (comp. Burgerstein, 1902). The analogies between these 

 movements and those of tendrils, especially the analogies with thermonastic 

 movements, are as we have already said, very great. They express themselves 

 in this, that during the movements due to stimulation, both incurvature and 

 recurvature, a new stimulus is always released owing to elevation of tempera- 

 ture ; the plants do not become accustomed to the stimulus, or, at least, do so only 

 gradually, for we can continue to induce opening movements with appropriate 

 elevations of temperature for many hours (Jost, 1898). All the same we must 

 not forget that there are differences to be taken account of. In the tendril 

 the most vigorous growth acceleration always takes place on the upper side 

 whether temperature be increased or decreased ; in the floral-leaves, on the 

 other hand, the upper side becomes convex when the temperature is raised 

 and the under side when the temperature is decreased. 



Not all flowers respond with nyctitropic movements as Tulipa and Crocus 

 do, exclusively or especially, when the temperature is altered ; some respond 

 only to alterations in light, others exhibit nyctitropic movements only when 

 light and temperature are altered at the same time. [As to other factors in 

 nyctitropic movements compare Hensel (1905).] Among plants which are 

 sensitive to alterations in light we must place the Compositae, where shading 



