6 Esther Pearl Hensel 



of the flower. The reason that MirabiHs closes earlier than Con- 

 volvulus is because it is easier for Mirabilis to fill its fibrous tis- 

 sue with oxygen under the influence of light and heat. In the 

 case of flowers opened and closed for several days, as the dande- 

 lion, etc., the fibrous tissue becomes gradually filled with oxygen 

 during the day when the flower is open. At the same time, the 

 sap current is diminished because of the decreasing light, thus 

 decreasing turgescence ; in consequence, the cellular tissue curves 

 inward and the flower closes. 



Hermann Hoffmann (1850) brought together results showing 

 that temperature was the all-important factor in opening and 

 closing, light influencing the processes only as it contained heat 

 rays. His experiments were conducted upon foliage leaves of 

 Oxalis tetraphylla and Mimosa pudica, and the flowers of Tolpis 

 harhata, Oenothera lindleyana, Onagra biennis, Lotus peregrinus, 

 Jpomoea purpurea, and Eschscholtzia. These were the principal 

 species experimented upon ; several others, however, were used 

 to help disprove that sleep movements are caused by moisture in 

 the air, electricity, or the expansion of gas within the plant — 

 causes to which opening had been ascribed. He further proved 

 that opening can be caused artificially at the hour of most pro- 

 found sleep by simple increase of heat without the aid of light, 

 but that the prolongation or excess of heat caused sleep. He 

 stated also that the dilatation of the sap by the action of heat 

 could not cause the daily expansion of a flower, since water ex- 

 pands only 1/22 of its volume between 0° and 100° C. 



Royer (1868) affinTied that variations of heat and turgescence 

 — complements of each other — were the cause of all flower move- 

 ments. Taraxacum taraxacum. Crocus, Tnlipa gcsneriana, Fi- 

 caria ranunculoides, and Bellis percnnis were experimented upon 

 by him as examples of sleeping flowers, i. e., those that open and 

 close several times. He made a close distinction, however, be- 

 tween sleeping flowers and ephemeral ones, such as Convolvulus 

 arvensis, C. sepinm, Glaucium Havum, Stelloria media, and sev- 

 eral Veronicas, which sleep only in appearance and close only 

 when their existence is ended. These latter, he said, could be 

 transformed into sleeping flowers by humid earth, shade, late 



196 



