OBSERVATIONS O^ FLOWERING PERIODS OF CERTAIN CACTI. 35 



frequently wait till the sun is well above the horizon before 

 closing. A photograph was taken with the advantage of shadows 

 cast by him, while one of the flower of Cereus thurheri had to 

 be made before the dawn was dispelled. The fact is that the 

 pitalla flowers possess nothing of the evanescent appearance of 

 the night blooming kinds, including also many other plants of 

 widely different families. Yet its thick and stiff petals, like to 

 those of Cereus giganteus, are quite as truly evanescent. The 

 explanation must- be sought by physiological methods. This 

 brings me to the consideration of another plant, which exempli- 

 fies, among the cacti, a third type of diurnal flowering period as 

 yet unrecognized, namely Opuntia fulgida. I shall speak now 

 of this alone, though what I say is true in part of several other 

 species more or less closely related to it. The observations on 

 these, however, are set down below. Opuntia fulgida differs 

 from both the night blooming and day blooming kinds, in that its 

 diurnal flowering period commences at three, suntime, in the 

 afternoon. So regular and constant is this that one may set 

 his watch by it and be scarcely fifteen minutes away from the 

 true time. This was illustrated on the occasion of a visit to 

 Tucson of Dr. Pehr Olsson-Seffer, to whom I mentioned the fact. 

 Upon this I took out my watch, which read 2:30 p. m. (3:05 

 sun time), and I suggested that we pass into the adjoining cactus 

 garden, ])lanted by Professor J. W. Toumey, to see if we might 

 verify my statements. L'^]:)on doing so, not an open flover was 

 to be seen. I was, however, able to point out what was not very 

 obvious to one who had not given attention to the point, that cer- 

 tain flowers would be out in fifteen minutes, that is, would l)egin 

 to open. We passed around the garden examining cither forms 

 here and there, when we came upon an opening flower, and the 

 watch had advanced fifteen minutes. We immediately returned 

 tO' examine the first ones we had looked at, and these, too, were 

 beginning to open ! In fifteen minutes more a dozen or two 

 flowers could be seen dotting the pendulous branches here and 

 there. Others as well as myself have, during the ])ast summer, 

 scaiiiu'(l large stretches of country, as one may in two long days' 



