MOVEMENTS DUE TO CHANGES IN THE INTENSITY OF LIGHT. 625 



Similarly with many flowers, the corollas of which open in the morning, rarely 

 in the evening, while they close again in the evening or morning respectively, definite 

 hours not being followed as a rule. 



These are the phenomena with which we shall now be concerned more in 

 detail. One of their characteristics is that they vary with the alternations of day and 

 night, and thus constitute daily periods. Now since in the morning, when the leaves 

 and flowers open, the temperature and dryness of the air usually increase, and in 

 the evening when they close the temperature falls, and the moisture of the air there- 

 fore increases, it might be supposed at first that these points must be of special 

 importance for the movements of sleeping and waking of leaves and flowers. As 

 a matter of fact, the opening and closing of many flowers, as the Tulip and Crocus, 

 are directly dependent on changes of temperature; and it cannot be denied that 

 in other cases also changes in temperature and moisture affect the phenomena to a 

 more or less subordinate extent. Nevertheless, so much is certain, that the daily 

 periodic movements are called forth chiefly and almost exclusively by alterations 

 in the brightness of the light, particularly in the morning and in the evening : the 

 nocturnal position of the leaves and flowers is due to the darkness which supervenes as 

 the sun goes down, and the diurnal position is equally a consequence of the morning 

 brightness. That temperature and moisture are entirely subordinate and unim- 

 portant in the matter is conclusively shown by means of a very simple experiment. 

 A small plant of the common Garden Bean {Phaseolus) or of the Wood Sorrel 

 {Oxalis), rooted in a flower-pot, and with the leaves extended in the diurnal position 

 in the forenoon, may be completely submerged in a large glass vessel filled with 

 water. If the illumination remains approximately as before, the leaves maintain 

 their diurnal position, although the water in which they are submerged is many 

 degrees colder than the air which previously surrounded them, whence it follows 

 that neither change of temperature nor of moisture produces an observable alteration 

 in the diurnal position. If, however, such experimental plants, when they have 

 assumed their diurnal position — it matters not whether they are in the open air or 

 under water — should be suddenly darkened, e.g. by covering them with an opaque 

 box, or with a cylinder of wood or card-board, the leaves assume the nocturnal 

 position after a short time, say half an hour to an hour ; and on again letting in the 

 light during the day, the leaves would again take up their ordinary diurnal position. 

 By means of such simple experiments therefore we can convince ourselves con- 

 clusively of the fact that the matter is one of stimulations due to variations in the 

 light — illuminating and darkening. 



With the establishment of this fact, however, we have for the first time obtained 

 a foundation on which a deeper insight into these phenomena may be based by 

 means of further research. Dwelling for the moment on the most general relations, 

 it is above all to be insisted upon that, with respect to organisation, it is always 

 organs with dorsi-ventral structure which are here concerned, as is clear from the 

 -fact that we have to do in all these cases with true leaves, or at least with metamor- 

 :phosed foliar organs, which always possess dorsi-ventral structure— i. e. the parts 

 capable of curving and of receiving the light-stimulus are organised on the lower 

 side more or less differently than on the upper side. Since then all these 

 organs, simply because they are leaves, are situated on a shoot-axis, and since the 



[3] ss 



