±22 MODIFIED CIECUMNUTATION. 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 bien made when wo were 



S-S not attending to heliotropism ; and they led 



T'2 us to experiment on several kinds of seed- 



_x4 lings, by exposing them to a dim lateral light, 



■g '^ so as to observe the gradations between 



^ S ordinary circumnutation and heliotropism. 



|;Sf Seedlings in pots were placed in front of, 



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



■g :S on each side and over the potR 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 



■S ^ window. By hanging up one or more blinds 



a o before the window where the seedlings stood, 



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



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



"o g- opposite one, which received the diffused 



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



^m blinds were successively removed, and as the 



-g g plants had been subjected during the day to 



g, g a very obscure light, they continued to bend 



^ a towards the window later in the evening than 



"o ■? would otherwise have occurred. Most of the 



bo 



■g-^ seedlings were selected because they were 



a 'I known to be highly sensitive to light, and 



'Z T, some because they were but little sensitive, 



^ ^ . or had become so from having grown old. 



'a a ■« The movements were traced in the usual 



/ ii-a^ manner on a horizontal glass cover; a fine 



^ § .S glass filament with little triangles of paper 



"^ -^ 'G having been cemented in an upright position 



gS's to the hypocotyls. Whenever the stem or 



■| ^ 'S hypocotyl became much bowed towards the 



I ■« 5 light, the latter part of its course had to 



"*■! a be traced on a vertical glass, parallel to the 



•1 " window, and at right angles to the horizontal 



'=5 glass cover. 

 A^s graveokns. — The hypocotyl bends in a few hours rectan- 



