CThap. VII. SLEEP OF LEAVES. 859 



In the diagram given on the next page (Fig. 150), the two 

 curved broken lines at the base, which represent the nocturnal 

 courses, ought to bo prolonged far downwards. On the first 

 day the leaflet moved thrice down and thrice up, and to a con- 

 siderable distance laterally ; the course was also remarkably 

 crooked. The dots were generally made every hour; if they 

 had been made every few minutes all the lines would have been 

 zigzag to an extraordinary degree, with here and there a loop 

 formed. We may infer that this would have been the ca^e, 

 because five dots were made in the course of 31m. (between 

 12.34 and 1.5 p.m.), and we see in the upper part of the diagram 

 how crool;ed the course here is ; if only the first and last dots 

 had been joined we should have had a straight line. Exactly 

 the same fact may be seen in the lines representing the course 

 between 2.24 p m. and 3 p.m., when six intermediate dots were 

 made ; and again at 4.46 and 4.50. But the result was widely 

 different after 6 p.m., — that is, after the great nocturnal descent 

 had commenced ; for though nine dots were then made in the 

 course of 82 m , when these were joined (see Figure) the line thus 

 formed was almost straight. The leaflets, therefore, begin to 

 descend in the afternoon by zigzag lines, but as soon as the 

 descent becomes rapid their whole energy is expended in thus 

 moving, and their course becomes rectilinear. After the leaflets 

 are completely asleep they move very little or not at all. 



Had the above plant been subjected to a higher temperature 

 than 67° — 70° F., the movements of the termiual leaflet would 

 probably have been even more rapid and wider in extent than 

 those shown in the diagram ; for a plant was kept for some time 

 in the hot-houfe at from 92° — 93° F., and in the course of 35 m. 

 the apex of a leaflet twice descended and once ascended, travelling 

 over a space of 1'2 inch in a vertical direction and of '82 inch in 

 a horizontal direction. Whilst thus moving the leaflet also 

 rotated on its own axis (and this was a point to which no atten- 

 tion had been before paid), for the plane of the blade differed by 

 41° after an interval of only a few minutes. Occasionally the 

 leaflet stood still for a short time. There was no jerking move- 

 ment, which is so characteristic of the little lateral leaflets. A 

 sudden and considerable fall of temperature causes the terminal 

 leaflet to sink downwards ; thus a cut-off leaf was immersed in 

 water at 95° F., which was slowly raised to 103° F., and after- 

 wards allowed to sink to 70° F., and the sub-petiole of the ter- 

 minal leaflet then curved downwards. The water was afterwards 

 24 



