i22 



MODIFIED CIKCTJMNUTATION. Chap. VIII. 



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overcast with extraordinarily dark thunder-clouds, and it was 

 interesting to note how plainly the cotyledons ciroumnutated 

 during this interval. 



The foregoing observations are of some 

 Fig. 170. value, from having b. en made when we were 



not attending to heliotropism ; and they led 

 us to experiment on several kinds of seed- 

 lings, by e.^posing them to a dim lateral light, 

 so as to observe the gradations between 

 ordinary circumnutation and heliotropism. 

 Seedlings in pots were placed in front of, 

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

 on each side and over the yjots black boards 

 were placed ; in the rear the pots were open 

 to the diffused light of the room, which 

 had a second north-east and a north-west 

 window. By hanging up one or more blinds 

 before the window where the seedlings stood, 

 it was easy to dim the light, so that very 

 little more entered on this side than on the 

 opposite one, which received the diffused 

 light of the room. Late in the evening the 

 blinds were successively removed, and as the 

 plants had been subjected during the day to 

 a very obscure light, they continued to bend 

 towards the window later in the evening than 

 would otherwise have occurred. Most of the 

 seedlings were selected because they were 

 known to be highly sensitive to light, and 

 some because they were but little sensitive, 

 or had become so from having grown old.. 

 The movements were traced in the usual 

 manner on a horizontal glass cover ; a fine 

 glass filament with little triangles of paper 

 having been cemented in an upright position 

 to the hypocotyls. "Whenever the stem or 

 hypoootyl became much bowed towards the 

 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. 

 Apios graveolens. — The hypocotyl bends in a few hours rectan- 



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