646 PLANT RESPONSE 



season, its turgescence being great, maintain its leaves in a 

 vertical position, but little affected by the action of light. 

 From this it will also become clear that any influence, such 

 as long maintenance under darkness, which modifies the 

 suctional activity, is liable to render the response of the plant 

 abnormal. 



4. Modification of effect by characteristic limits of flexi- 

 bility. — I shall here deal with another important factor in 

 the determination of the final position assumed by the 

 leaf — that is to say, with the anatomical peculiarities which 

 determine the limits of flexibility. Let us take the petiole 

 bearing the terminal leaflet of a leaf of Desmodium. We 

 now suppose this leaflet to be outspread, in such a position 

 that its midrib is in a continuous straight line with the 

 petiole. This upper line we shall know as the dorsal line. 

 It consists of two parts or components, the laminal and the 

 petiolar. In this particular position they form a straight 

 line ; but the movement of the leaflet takes place with the 

 point of junction as the hinge. We shall then distinguish 

 that particular position of the dorsal line in which the laminal 

 and petiolar halves are continuous and straight as neutral, 

 and angular movements above or below will be measured 

 accordingly. In the case of Desmodium, when the terminal 

 leaflet has reached the neutral position, it cannot, owing to 

 the anatomical peculiarity of the joint, be bent further up- 

 wards ; but it can be bent in the opposite direction, that is 

 downwards, until the leaflet lies along the under side of the 

 petiole, the curvature being then through 180 . Thus the 

 limits of flexibility of this leaflet may be expressed by the 



o° 

 formula - - : that is to say, its flexibility above the 

 180 y y 



neutral position is o°, and below, 180°. Now, the petiole 



itself, in the case of Desmodium, for example, may, and does, 



occupy many different positions with regard to the stem. 



Let us suppose it at a given moment to subtend an acute 



angle, being thus more or less vertical, and suppose the 



terminal leaflet to be horizontal. If light now acts from 



