i22 MODIFIED CIKCUMNUTATION. Chap. VIII. 



overcast with extraordinarily dark tliunder-clouds, and it was 

 interesting to note how plainly the cotyledons circumnutated 

 during this interval. 



The foregoing observations are of some 

 Kig. 170. value, from having been made when wo were 



S -S not attending to heliotropism ; and they led 

 ■g ■? lis to experiment on several kinds of seed- 

 jK J lings, by e-^posing them to a dim lateral light, 

 ■g " so as to observe the gradations between 

 .§ £ ordinary oircumnutation and heliotropism. 

 a;£f Seedlings in pots were placed in front of, 

 « and about a yard from, a north-east window; 



•g :3 on each side and over the pots black boards 

 ^ - were placed ; in the rear the pots were open 

 ;^g. to the diffused light of the room, which 

 S "^- had a second north-east and a north-west 



bo ^ 



II ^ window. By hanging up one or more blinds 



"Z o before the window where the seedlings stood, 



j3 -J it was easy to dim the light, so that very 



.5 o little more entered on this side than on the 



^ opposite one, which received the diffused 

 !5 < light of the room. Late in the evening the 

 i^m blinds were successively removed, and as the 

 •g g plants had been subjected during the day to 



I p, S a very obscure light, they continued to bend 



r n towards the window later in the evening than 



; .2| would otherwise have occurred. Most of the 



i-^ seedlings were selected because they were 



i g known to be highly sensitive to light, and 



; -s some because they were but little sensitive, 



' ■" ^ or had become so from having grown old. 



\ g" a "S ^'^^ movements were traced in the usual 



) ^-e^ manner on a horizontal glass cover; a fine 



=3 S .9^ glass iilament with little triangles of paper 



__ "^^'g having been cemented in an upright position 



§■1)° *'° ^^^ hypocotyls. Whenever the stem or 



"t ^ "H hypocotyl became much bowed towards the 



1 "g 5 light, the latter part of its course had to 

 ^J I be traced on a vertical glass, parallel to the 

 •s window, and at right angles to the horizontal 

 '^ glass cover. 



Apios graveolcns. — The hypocotyl beads in a few hours rectan- 



