338 * MODIFIED CIECUMNUTATION. Chap. VIJ 



least, and did not close at all duri^ the hottest days. Anothei 

 trial was made with the same bush on August 2nd and 6th (the soil 

 appearing at this latter date extremely dry), for it was exposed 

 out of doors during the whole day to the wind, but the leaflets 

 showed no signs of closing. The Chilian form therefore differs 

 widely from the one at Wurzburg, in not closing its leaflets 

 when suffering from the want of water; and it can live for a 

 surprisingly long time without water. 



Tropwolum mujus (?) (cultivated var.) (TropEeoleae). — Several 

 plants in pots stood in the greenhousp, and the blades of 

 the leaves which fatted the front-lights were during the day 

 highly inclined and at night vertical ; whilst the leaves on 

 the back of the pots, though of course illuminated through 

 the roof, did not become vertical at night. We thought, at first, 

 that this difference in their positions was in some manner 

 due to heliotropism, for the leaves are highly heUotropic. The 

 true explanation, however, is that unless they are well illu- 

 minated during at least a part of the day they do not sleep at 

 night; and a little difference in the degree of illumination deter- 

 mines whether or not they shall become vertical at night. We 

 have observed no other so well-marked a case as this, of the 

 influence of previous illumination on nyctitropic movements. 

 The leaves present also another peculiarity in their habit of 

 rising or awaking in the morning, being more strongly fixed or 

 inherited than that of sinking or sleepiug at night. The move- 

 ments are caused by the bending of an upper part of the petiole, 

 between J and 1 inch in length ; but the part close to the blade, 

 for about i of an inch in length, does not bend and always 

 remains at right angles to the blade. The bending portion does 

 not present any external or internal difference in structure 

 from the rest of the petiole. We will now give the experiments 

 on which the above conclusions are founded. 



A large pot with several plants was brought on the morning 

 of Sept. 3rd out of the greenhouse and placed before a north-east 

 window, in the same position as before with respect to the light, 

 as far as that was possible. On the front of tlie plants, 24 Jeaves 

 were marked with thread, some of which had their blades hori- 

 zontal, but the greater number were inclined at about 45°, 

 beneath the horizon; at night all these, without exception, 

 became vertical. Early on the following morning (4th) they 

 reassumed their former positions, and at night again became 

 vertical. On the 5th the shutters were opened at 6.15 a.m.. and 



