476 TRANSMITTED EFFECTS OF LIGHT. CHAP. IX 



crack or furrow all round their bases, which would admit a 

 little light on all sides ; but this would not happen when they 

 were illuminated laterally, for we know that they quickly bend 

 towards a lateral light, and they then press so firmly against the 

 sand on the illuminated side as to furrow it, and this would 

 effectually exclude light on this side. Any light admitted on 

 the opposite and shaded side, where an open furrow is formed, 

 would tend to counteract the curvature towards the lamp or 

 other source of the light. It may be added, that the use of fine 

 moist sand, which yields easily to pressure, was indispensable 

 in the above experiments ; for seedlings raised in common soil, 

 not kept especially damp, and exposed for 9 h. 30 m. to a strong 

 lateral light, did not form an open furrow at their bases on the 

 shaded side, and were not bowed beneath the surface. 



Perhaps the most striking proof of the action of the upper 

 on the lower part of the cotyledons of Phalaris, when laterally 

 illuminated, was afforded by the blackened glass-tubes (before 

 alluded to) with very narrow stripes of the varnish scraped 

 off on one side, through which a little light was admitted. 

 The breadth of these stripes or slits varied between '01 and 

 02 inch ('25 and *51 mm.). Cotyledons with their upper 

 halves enclosed in such tubes were placed before a south-west 

 window, in such a position, that the scraped stripes did not 

 directly face the window, but obliquely to one side. The seed- 

 lings were left exposed for 8 h., before the close of which time 

 the many free seedlings in the same pots had become greatly 

 bowed towards the window. Under these circumstances, the 

 whole lower halves of the cotyledons, which had their summits 

 enclosed in the tubes, were fully exposed to the light of the 

 sky, whilst their upper halves received exclusively or chiefly 

 diffused light from the room, and this only through a very 

 narrow slit on one side. Now, if the curvature of the lower 

 part had been determined by the illumination of this part, all 

 the cotyledons assuredly would have become curved towards 

 the window; but this was far from being the case. Tubes 

 of the kind just described were placed on several occasions 

 over the upper halves of 27 cotyledons ; 14 of them remained 

 all the time quite vertical; so that sufficient diffused light 

 did not enter through the narrow slits to produce any effect 

 whatever; and they behaved in the same manner as if their 

 upper halves had been enclosed in completely blackened tubes. 

 The lower halves of the 13 other cotyledons became bowed 



