122 THE BIOLOGY OF FLOWERING PLANTS 



curvature ; they are little understood. Again, the change 

 in position is brought about, frequently by a combination of 

 reactions, at the base of the blade, in the stalk, and even in 

 the shoot. Finally, the leaf blade takes up its position at an 

 angle to the direction of the controlling illumination — it is 

 a plagiotropic, not an orthotropic, organ. Although the 

 reaction is carried out in response to light it is influenced 

 by gravity, so that some leaves, at least, assume a horizontal 

 position even in absence of light. 



The case of the sunflower showed us the petioles 

 inchned rather steeply above, and the blades steeply below, 

 the horizontal. The position of the leaf is thus attained by 

 the movement of petiole and blade (or the point of junction 

 of blade and petiole) in different directions. Is, then, the 

 stimulus perceived by one or by both organs, and how is 

 the ultimate adjustment brought about ? These questions 

 must be answered differently for different plants. Haber- 

 landt (1905) has shown that there are three main types of 

 behaviour, connected by intermediate types : 



(i) In Begonia discolor and Monstera deliciosa the leaf 

 blade alone perceives the stimulus, which is transmitted to 

 the petiole, in which the appropriate movements are carried 

 out. 



(2) In the simple primary leaves of the scarlet runner 

 the petiole and the pulvinus, which is here present, perceive 

 the stimulus directly, and bring the leaf blade into the 

 favourable position. The blade is little sensitive, although 

 it may have a certain influence. 



(3) In most plants both petiole and blade are sensitive. 

 Either, illuminated by itself, brings the blade into the 

 proper position ; where the blade is illuminated the stimulus 

 must of course be transmitted. The petiole, when 

 illuminated alone, can bring about an approximate adjust- 

 ment only. Illumination of the blade alone produces more 

 perfect adjustment. The indirect stimulus through the 

 blade can overcome the direct stimulus to the petiole. 

 Haberlandt makes the acute suggestion that the direct 

 stimulus of the petiole secures a " coarse " adjustment, while 



