?7G TRANSMITTED EFFECTS OF IJGHT. Chav. IX 



ci'ack or furrow all riiunrl their bases, which would admit a 

 little liglit on all siilc.s; but this would not happen when thej 

 were illuminated laterally, foi' we know that thoy quickly bend 

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

 sand on the illuminated side as to furi'ow it, and this would 

 effectually exclude light on this side. Any liglit admitted on 

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

 would tend to counteract the curviituro towards the lamji or 

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

 moist sand, whicli 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 furi-ow 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 uart of flic cotyledons of Plialaris, when laterally 

 illuminated, was afforded by the blackened glass-tnlios (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 ('2') and '!>! mm.). Cotyledons with their U)))ier 

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

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

 directly l^ice the window, but obliquely to one side. The seed- 

 lings wore left exposed for 8 h., l)efore the close of which time 

 the many frro seedlings in the same pots had become greatly 

 bowed towards the window. Under those circumstances, the 

 whole lower halves of the cotyledons, which had their sumn\its 

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

 sky, whilst their upper halves receiv(!d 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 Io^mt 

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

 the cotyledons assuredly would liave 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 ; 11 of tluim remained 

 all the time quite vertical; so that sulfieiont diffused light 

 did not enter tiirough the narrow slits to produce any effect 

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

 upper halves liad liern enclosed in completely blackened tubes. 

 The lower halves of tlie 13 other cotvledons l)0fMrae bowed 



