Fig. 162.  Nicotiana glauca:  shoots with leaves expanded during the day, and asleep at night.  Figures copied from photographs, and reduced.

of the species which must be well illuminated during the day in order to sleep, for on two occasions when plants were kept all day in a room with north-east windows, the leaves did not sleep at night.  The same cause probably accounts for the leaves on our seedlings raised in the dead of the winter not sleeping.  Professor Pfeffer informs us that the leaves of another species (S.  Jorullensis ?) hang vertically down at night. [page 386]

Ipomoea caerulea and purpurea (Convolvulaceae).—­The leaves on very young plants, a foot or two in height, are depressed at night to between 68o and 80o beneath the horizon; and some hang quite vertically downwards.  On the following morning they again rise into a horizontal position.  The petioles become at night downwardly curved, either through their entire length or in the upper part alone; and this apparently causes the depression of the blade.  It seems necessary that the leaves should be well illuminated during the day in order to sleep, for those which stood on the back of a plant before a north-east window did not sleep.

Nicotiana tabacum (var.  Virginian) and glauca (Solaneae).—­The young leaves of both these species sleep by bending vertically upwards.  Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are given on p. 385:  one of the shoots, from which the photographs were taken, was accidentally bent to one side.

Fig. 163.  Nicotiana tabacum:  circumnutation and nyctitropic movement of a leaf (5 inches in length), traced on a vertical glass, from 3 P.M.  July 10th to 8.10 A.M. 13th.  Apex of leaf 4 inches from glass.  Temp. 17 1/2o — 18 1/2o C. Figure reduced to one-half original scale.

At the base of the petiole of N. tabacum, on the outside, there is a mass of cells, which are rather smaller than elsewhere, and [page 387] have their longer axes differently directed from the cells of the parenchyma, and may therefore be considered as forming a sort of pulvinus.  A young plant of N. tabacum was selected, and the circumnutation of the fifth leaf above the cotyledons was observed during three days.  On the first morning (July 10th) the leaf fell from 9 to 10 A.M., which is its normal course, but rose during the remainder of the day; and this no doubt was due to its being illuminated exclusively from above; for properly the evening rise does not commence until 3 or 4 P.M.  In the figure as given on p. 386 (Fig. 163) the first dot was made at 3 P.M.; and the tracing was continued for the following 65 h.  When the leaf pointed to the dot next above that marked 3 P.M. it stood horizontally.  The tracing is remarkable only from its simplicity and the straightness of the lines.  The leaf each day described a single great ellipse; for it should be observed that the ascending and descending lines do not coincide.  On the evening of the 11th the leaf did not descend quite so low as usual, and it now zigzagged a little.  The diurnal sinking movement had already commenced each morning by 7 A.M.  The broken lines at the top of the figure, representing the nocturnal vertical position of the leaf, ought to be prolonged much higher up.