OPENING OF THE PASSAGE TO THE INTERIOR OF THE FLOWER. 219 



The times of opening and closing given here and in the Floral Clock apply only 

 to fine days. In cloudy, misty, and rainy weather, the flowers remain closed or 

 only partly open; or, when these conditions are but temporary, a conspicuous 

 retardation of opening and closing takes place, which cannot, however, be indicated 

 numerically. Further, the observations given above relate in particular to plants 

 well placed in regard to illumination. Such a limitation of the flowers under 

 observation is absolutely essential, if tolerably reliable results are required. The 

 greatest care is necessary, especially in the case of flowers which open quickly. 

 Thus one finds with the almost suddenly-opening Gentians (Gentiana ciliata, 

 Bavarica, and verna), that whilst those growing on the east or south side of a 

 hillock have already opened wide their flowers, those a few yards away with 

 a northerly aspect still keep their flowers closed. On 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. 



The whole of these observations point to the fact that the opening of flowers 

 is especially promoted by sunshine. Exactly how it is brought about, how the 

 influence of the sun's rays in these opening-movements of flowers affects the 

 tension of the tissues, is by no means easy of explanation. Still the question 

 is so full of interest that it is well worth our consideration. First, we may 

 inquire whether it is light or heat which gives the impulse to the remarkable 

 changes in tension which lead to the movements of the petals. Plants were 

 employed, for the solution of this question, whose flowers open directly they are 

 reached by the first morning rays of sunshine. Specimens of Gentiana Rhcetica 

 and asclepiadea were placed in a roomy cylinder of glass in which the temperature 

 was maintained at a low and uniform temperature. This was effected by surround- 

 ing the cylinder by a second, larger one, and causing a stream of water of a 

 constant temperature of 7 C. to circulate in the space between the cylinders. 

 Since this mantle of water only permitted rays of light and not of heat to pass, it 

 would be due solely to the action of the former if the flowers under experiment 

 opened. As the rays of the morning sun reached the cylinder the Gentians within 

 opened their flowers. In view of this result one is justified in assuming that the 

 opening is occasioned by the rays of light. But that it arises solely from this cause 

 were too hasty a conclusion, as appears from the following control-experiment, 

 conducted upon the same Gentians. They were, whilst closed, placed in a dark 

 room over the still hot iron of a stove in a situation where the thermometer indi- 

 cated 42 C. Within 3 minutes they had all completely opened. 



This apparent contradiction may be explained by the assumption that the rays 

 of light which fell upon the closed Gentian flowers in the cylinder were converted 

 into heat. As we know, if rays of light strike any object and are not entirely 

 reflected from its surface, they warm it (cf. vol. i. p. 519). This probably is the case 

 with the Gentians, and the phenomenon may be explained as follows. The light- 

 vibrations are communicated to the flowers and converted into vibrations of heat. 

 This heat produces changes in the turgidity of the tissues, affecting their tension 



