oh THE PLANT WORLD. 



drive, and never an open flower of this plant has been seen before 

 3 :00 p. m. by the sun, and our search has inchided very many 

 thousands of plants. 



It must be observed that nearly all these observations were 

 made in brieht weather, as there is little of anv other kind in 

 this part of Arizona. One observation, communicated to me by 

 Prof. J. J. Thornber, made by him on the mesa a few miles 

 southeast of Tucson on a very cloudy day, indicates that the open- 

 ing,' mav be induced earlier than normallv bv continuous reduced 

 illumination. He saw the flowers of Opuntia fulgida and 0. 

 mamiUata opening at exactly 2 :00 p. m., a half hour earlier 

 than usual. The two species behaved alike. In thirty minutes 

 flowers in 0]>en condition were abundant, so tliat this is probably 

 generally true of these plants on cloudy days. It is well known 

 that changes in illumination induce the opening or closing of 

 certain flowers, and this applies also in many cases to changes of 

 temperature. The immediate cause of opening may be growth 

 or turgor changes, or both. In the case of the plant of which I 

 am speaking, it seems difficult to account for the simultaneous 

 opening of the flowers with such great regularity as a result of 

 any change in the illumination or temperature. Both of these 

 factors are remarkably constant during the hours preceding and 

 following the beginning of the diurnal flowering period ; nor are 

 we in a position to state that the change in the quality of illumi- 

 nation is at all marked at this time. Of the very slight changes, 

 insensible to our crude organs, which may well be believed to 

 affect the flowers of ])lants, we cannot do more than to conjecture. 

 Xo ordinary means that we have at hand can record them. That 

 the opening of these flowers is a response to such changes or is an 

 after-effect of previous conditions, or how far it is conditioned 

 by factors, such as water content, within the plant itself, are 

 open questions and oft'er a fertile field for study. Kerner (]^at- 

 ural History or Plants II., P. 219) remarks: "In luxuriant 

 Opuntia plants it is quite common for the flowers on the branches 

 of the sunny side to open a long time before those on the shady 

 side, and this with flowers of the same age." I have failed to 



