256 THE JOUENAL OF BOTAJTY 



pollen-eating, and nectar-stealing insects, especially ants, which 

 cannot travel until the dew is oft" the herbage. This applies no doubt 

 to some species of plants ; but it seems not to apply to the case of 

 the Trayopogon, which in our latitude remains open usually till 

 about noon, by which time as a rule the dew has long been oft" the 

 herbage. 



A much more likely theory is that the open-hours of all these short 

 period flowering plants is governed, in some way, by the access of the 

 sun's light and heat. Experiments made by Kerner (see his Nat. 

 Hist, of Plants, ii. 219-221 ; 1908) have shown, indeed, that this is 

 really the main governing cause of the phenomena presented by the 

 flowers of these plants. At all events, it appears to explain adequately 

 why the open-hours of the flowers of the Tragopogons should be 

 from three to three-and-a-half hours earlier at Upsala than in the 

 valley of the Inn, some thirteen degrees farther south, and six degrees 

 farther west ; for the sun rises earlier in the more northerly locality 

 than in the more southerly. 



Yet this theory by no means explains all the problems involved. 

 That some flowers should thus "go to sleep" (as village children 

 express it) for a certain portion of each day whilst others beep 

 "awake" (so to speak) the whole day is comparatively easy of ex- 

 planation ; for flowers attain their chief object (namely, their pollina- 

 tion) by many different means — some by one, some by another. 



A problem much more difficult of solution is that involved in the 

 • piestion why different species of these "sleeping" flowers should 

 differ so remarkably in their waking or open-hours, when all are 

 exposed, in the same degree, to the light and heat of the sun. 

 Lubbock raised this question nearly half-a-century ago (Flowers 

 in relation to Insects, p. 21; 1S75), but he failed to answer it; and, 

 so far as I am aware, no one has answered it satisfactorily since his 

 time. 



It may be urged, with a high degree of probability, that the 

 opening and closing of most flowers is governed, in some way or 

 other, by the angle at which the sun's rays strike the flower. In the 

 case of these "sleeping" flowers, it will he found that most, if not 

 all, open directly upwards, thus rendering them specially liable to be 

 acted upon in this way. 



But these theories, even if accepted fully, do not solve — on the 

 contrary, they raise — the problem: — How r is it that some flowers 

 which " sleep " do so at one period of the day whilst others do so at 

 another time of day, though all alike must receive the rays of the 

 sun at the same angle and in the same degree ? 



One can only suggest, by way of solution, that the petals of 

 these sleeping plants may be provided with some mechanism, more or 

 less analogous to the " eye " of the fungus Pitobo/us, lately described 

 by Prof. Reginald Buller (see Trans. Brit. Mvcol. Soc. vii. 61; 

 Journ. Bot. 1921, 206), which causes each of these flowers to open or 

 close (as the case may be) when the sun's rays happen to fall upon it 

 at some particular angle. 



P.S. — Since the foregoing was put into type, I have been informed, 

 by the kindness of the Assistant-Secretary of the Royal Astronomical 



