122 MODIFIED CIRCUMNUTATION. CHAP. VIIL 



overcast with extraordinarily dark thunder-clouds, and it was 

 interesting to note how plainly the cotyledons circumnutated 

 during tliis interval. 



The foregoing observations are of some 

 Fig. 170. value, from having bten made when we were 



^-2 not attending to heliotropism ; and they led 

 ]j"S us to experiment on several kinds of seed- 

 j>.| Ii n 8' s j by exposing them to a dim lateral light, 

 g 2 so as to observe the gradations between 

 ,| 2 ordinary circumnutation and heliotropism. 

 Q.J& Seedlings in pots were placed in front of, 

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



"25 on each side and over the pots black boards 

 were placed ; in the rear the pots were open 

 *^|L, to the diffused light of the room, which 

 Sf r ^ had a second north-east and a north-west 

 j5 ^ window. By hanging up one or more blinds 

 a o before the window where the seedlings stood, 

 ^2 it was easy to dim the light, so that very 

 *o little more entered on this side than on the 

 *= a opposite one, which received the diffused 

 t? light of the room. Late in the evening the 

 ~^n blinds were successively removed, and as the 



t^ plants had been subjected during the day to 

 g a very obscure light, they continued to bend 

 j?^ towards the window later in the evening than 

 *S 3 woiild otherwise have occurred. Most of the 

 g^ seedlings were selected because they were 

 known to be highly sensitive to light, and 

 I some because they were but little sensitive, 

 * j or had become so from having grown old. 

 'I'fl'S T lie movements were traced in the usual 

 -|^-^ manner on a horizontal glass cover; a fine 

 ^ | .5 glass filament with little triangles of paper 

 M **/ having been cemented in an upright position 

 3 to the hypocotyls. Whenever the stem or 

 "2 hypocotyl became much bowed towards the 

 3 light, the latter part of its course had to 

 be traced on a vertical glass, parallel to the 

 window, and at right angles to the horizontal 

 glass cover. 

 Apio* graveolens.The hypocotyl bends in a few hours vectan- 



