IRRITABILITY. 447 



the sun's rays only when the sun is high in the heavens, 

 the leaves present their superior surfaces to the incident rays. 

 Thirdly, if the plants are so situated that they receive only 

 the oblique rays of either the morning- or the evening-sun, 

 the leaves place their superior surfaces at right angles to the 

 incident rays. The conclusion to be drawn from these observa- 

 tions is this, that it is the oblique rays of the sun which deter- 

 mine the vertical and meridian position of the leaves. The 

 presentation of the upper surface of any leaf to the east or 

 to the west is determined by the illumination to which its 

 upper surface is exposed. Thus, the leaves borne on the east 

 side of the stem have the sun's rays falling upon their upper 

 surfaces principally during the afternoon, whereas those borne 

 on the west side receive the rays of the sun upon their upper 

 surfaces chiefly in the morning : hence the former direct their 

 upper surfaces towards the west, and the latter towards the 

 east. 



The fixed light-position of most leaves is determined, as 

 Wiesner has shewn, not by the direction of incidence of direct 

 sunlight, but by the direction of incidence of the brightest 

 diffuse daylight. He observed, for instance, that when plants 

 were so situated that they received direct sunlight only for 

 a time in the morning and diffuse daylight during the rest of 

 the day, their fixed light-position was such that their upper 

 surfaces were directed perpendicularly to the direction of 

 incidence of the daylight and not to that of the rays of the 

 morning-sun. In this respect the leaves of the Compass-plants 

 behave differently. When exposed only to diffuse light, falling 

 from above, they are horizontal, and it is not until they are 

 exposed to direct sunlight that they take up the characteristic 

 vertical meridian position. This difference in behaviour can 

 only be accounted for by attributing to the two kinds of leaves 

 in question differences in irritability. Diffuse daylight is in- 

 sufficient to induce the assumption of their peculiar fixed 

 light-position by the leaves of the Compass-plants, direct 

 sunlight is necessary; in other plants, on the contrary, the 

 fixed light-position is determined by the direction of incidence 

 of the brightest diffuse daylight. 



