CHAP. IX. TRANSMITTED EFFECTS OF LIGHT. 479 



the tree seedlings. These latter four cases, as well as the t\vo 

 in the last paragraph, offer a strong exception to the rule that 

 the illumination of the upper part determines the curvature of 

 the lower part. Nevertheless, 5 of these 8 cotyledons remained 

 quite upright, although their lower halves were fully illuminated 

 all the time ; and it would almost be a prodigy to find five free 

 seedlings standing vertically after an exposure for several hours 

 to a lateral light. 



The cotyledons of Avena, like those of Phalaris, when growing 

 in soft, damp, fine sand, leave an open crescentric furrow on the 

 shaded side, after bending to a lateral light ; and they become 

 bowed beneath the surface at a depth to which, as we know, 

 light cannot penetrate. The arcs of the chords of the buried 

 bowed portions formed in two cases angles of 20 and 21 with 

 the perpendicular. The open furrows on the shaded side were, 

 in four cases, -008, '016, '024, and -024: of on inch in breadth. 



llrassim ohracea (Common Eed). It will here be shown that 

 the upper half of the hypocotyl of the cabbage, when illuminated 

 by a lateral light, determines the curvature of the lower half. 

 It is necessary to cxperimentise on young seedlings about half 

 an inch or rather less in height, for when grown to an inch and 

 upwards the basal part ceases to bend. We first tried painting 

 ihe hypoeotyls with Indian ink, or_cutting off their summits for 

 various lengths ; but these experiments are not worth giving, 

 though they confirm, as far as they can be trusted, the results 

 of the following ones. These were made by folding gold-beaters' 

 skin once round the upper halves of young hypoeotyls, and 

 painting it thickly with Indian ink or with black grease. As 

 a control experiment, the same transparent skin, left unpainted, 

 was folded round the upper halves of 12 hypoeotyls : and these 

 all became greatly curved to the light, excepting one which was 

 only moderately curved. Twenty other young hyponotyls had 

 the skin round their upper halves painted, whilst their lower 

 halves were left quite uncovered. These seedlings were then 

 exposed, generally for between 7 and 8 h., in a box blackened 

 within and open in front, either before a south-west window or 

 a paraffin lamp. This exposure was amply sufficient, as was 

 shown by the strongly-marked heliotropism of all the free seed- 

 lings in the same pots; nevertheless, some were left exposed 

 to the light for a much longer time. Of the 20 hynocotyls 

 thus treated, 14 remained quite upright, and 6 l>ecame sUerhtly 

 bowed to the light; but 2 of these hitter cases were not really 



