36 Esther Pearl Hensel 



fore its time by an extra amount of heat, with either dry or moist 

 air. It is impossible, however, to open an ephemeral flower by 

 placing the plant in a lower temperature, since this checks growth, 

 and opening is here rather a growth movement than a stimu- 

 latory one as in the other types. 



In the careful study by experiment of Taraxacum taraxacum, 

 Mentzelia nuda, Ipomoea purpurea, Linum usitatissimum, Oxalis 

 stricta, Mirabilis jalapa, and Pachylophus caespitosus, light, hu- 

 midity of the air, and water-content of the soil have been success- 

 fully eliminated as possible physical factors likely to cause the 

 opening and closing of flowers by the movement of the petals 

 (or florets). Heat, on the other hand, by its variations during 

 twenty-four hours, is the direct cause of movement in hemer- 

 anthous and nyctanthous types that bloom for more than one day. 

 In the case of those ephemeral flowers which open very early in 

 the morning before the temperature has risen to any extent, as 

 the morning glory, in contrast to those like purslane which open 

 as the temperature rises, or those like the evening primrose which 

 open a short time after the higher temperatures of the day have 

 given place to the lower ones of night, the phenomenon is not to 

 be explained so easily; it is possible that they react to a smaller 

 variation in temperature than do the others mentioned. 



The closing of ephemeral flowers is, however, a different proc- 

 ess from that of periodic flowers, since it signifies the end of the 

 existence of the flower. This closing, as has been shown, can be 

 delayed for several hours by a temperature constantly lower than 

 normal, showing that the two are closely connected. It would 

 seem to be a tenable theory that ephemeral types of flowers have 

 arisen by an extra need for protection of the flower against ex- 

 cessive heat and evaporation, e. g., in a dry or warm climate, for 

 it is true in nearly every instance that the ephemeral type of 

 flower either blooms at night or for only a few hours during the 

 day. The differentiation into these types in the past generations 

 must have come about in some such way, and they have persisted 

 because of this favorable adaptation. 



The cause of the periodic movements of hemeranthous and 

 nyctanthous types is, however, explainable through the influence 



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