168 DIAHELIOTROPISM. [CH. VII 



rolled, each like a cigarette paper, round the base of a pin 

 from which the head has been cut. The pipes so made 

 are closed by pinching them at one end with a forceps. 

 The pinched part should be about 3 mm. in length and 

 should be bent as well as pinched. The little hollow 

 cylinders so constructed can be picked up with a forceps 

 and slipped over the cotyledons of some 6 — 8 Setaria 

 seedlings; the pot should then be exposed td lateral 

 illumination. When time has been allowed for helio- 

 tropic curvature to occur, the contrast between the 

 capped seedlings, which remain upright, and the others, 

 which are sharply bent towards the light, is striking. 



Section D. Diaheliotropism, Diageotropism, 

 Epinasty, Nutation of Epicotyls. 



(216) Diaheliotropism or transverse heliotropism. 



Before proceeding to experiment in this subject, it is 

 well to study the positions naturally assumed by leaves 

 when obliquely illuminated. For this purpose plants 

 with decussate leaves are useful, e.g. Lamium alhwm, 

 Syringa vulgaris, and especially the shrubby Veronicas 

 such as V. traversi and salicifolia. The drawing, fig. 33, 

 represents the position of the leaves of the latter species, 

 the light being supposed to fall obliquely from the left. 

 It will be seen that the leaf T which points towards 

 the lighted side has a curve in its petiole which brings 

 the surface of the leaf at right angles to the light; 

 the leaf F is also at right angles to the light, but 



