1 6 Esther Pearl Hensel 



The reason why cloudiness has been considered by some in- 

 vestigators as an important factor in movement is probably be- 

 cause a reduction in the amount of light (cloudiness) is nearly 

 always accompanied by a lower temperature. The factors have 

 not been carefully separated. 



Movement can not be brought about by an increase of turgor 

 within the cells, for with a rise of 15° C. the pressure is only 

 raised from 100 to 105.5, ^^^ this, according to Pfeffer, could 

 not cause a movement. Fifteen degrees of temperature, Centi- 

 grade, are, in no case that I know of, necessary to effect open- 

 ing, and hence sufficient power could not be obtained in this way. 

 Experiments were made with flowers cut off from the plant to 

 determine whether turgescence was efficient in closing them. 

 Some were immersed in water, others in different per cents of 

 sugar solutions — all with the same results — opening and closing 

 at the usual times. 



It seems not at all probable that the expansion of the gases in 

 the plant could increase turgidity by the rise of temperature, and 

 hence cause opening, since, as far as temperature is concerned, 

 gaseous pressure and osmotic pressure are governed by the same 

 laws. Moreover, how can night blooming flowers be explained 

 by this theory ? Lack of time prevented experimentation on such 

 an improbable cause. 



That the protoplasm of the cells within the plant could be 

 stimulated by temperature is the last and only plausible explana- 

 tion. Just how this is done can not be understood, but that flow- 

 ers are positively or negatively thermotropic to certain tempera- 

 ture extremes seems evident. Why a certain flower is ephemeral 

 while others are hemeranthous or nyctanthous seems to me to 

 be only partially answerable. It is doubtless true that the char- 

 acter is inherent, but the original causes must have been climatic, 

 a flower closing for biological reasons, such as protection of pol- 

 len against unfavorable weather conditions, etc., and loss of 

 water b}' excessive heat. An attempt was made by Dutrochet 

 in 1836, and Farmer in 1902, to explain movement by means of 

 the corolla (of Taraxacum and Mirabilis in the case of the for- 

 mer, and of Tulipa in that of the latter). " Dutrochet gives tur- 



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