We may even infer as probable that the less or greater destruction during a frost of the leaves on a plant which does not sleep, may often depend on the greater or less degree of flexibility of their petioles and of the branches which bear them.
NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.
We now come to the descriptive part of our work, and will begin with cotyledons, passing on to leaves in the next chapter. We have met with only two brief notices of cotyledons sleeping. Hofmeister,* after stating that the cotyledons of all the observed seedlings of the Caryophylleae (Alsineae and Sileneae) bend upwards at night (but to what angle he does not state), remarks that those of Stellaria media rise up so as to touch one another; they may therefore safely be said to sleep. Secondly, according to Ramey**, the cotyledons of Mimosa pudica and of Clianthus Dampieri rise up almost vertically at night and approach each other closely. It has been shown in a previous chapter that the cotyledons of a large number of plants bend a little upwards at night, and we here have to meet the difficult question at what inclination may they be said to sleep? According to the view which we maintain, no movement deserves to be called
* 'Die Lehre von der Pflanzenzelle,' 1867, p. 327.
** 'Adansonia,' March 10th, 1869.
[page 298] nyctitropic, unless it has been acquired for the sake of lessening radiation; but this could be discovered only by a long series of experiments, showing that the leaves of each species suffered from this cause, if prevented from sleeping. We must therefore take an arbitrary limit. If a cotyledon or leaf is inclined at 60o above or beneath the horizon, it exposes to the zenith about one-half of its area; consequently the intensity of its radiation will be lessened by about half, compared with what it would have been if the cotyledon or leaf had remained horizontal. This degree of diminution certainly would make a great difference to a plant having a tender constitution. We will therefore speak of a cotyledon and hereafter of a leaf as sleeping, only when it rises at night to an angle of about 60o, or to a still higher angle, above the horizon, or sinks beneath it to the same amount. Not but that a lesser diminution of radiation may be advantageous to a plant, as in the case of Datura stramonium, the cotyledons of which rose from 31o at noon to 55o at night above the horizon. The Swedish turnip may profit by the area of its leaves being reduced at night by about 30 per cent., as estimated by Mr. A. S. Wilson; though in this case the angle through which the leaves rose was not observed. On the other hand, when the angular rise of cotyledons or of leaves is small, such as less than 30o, the diminution of radiation is so slight that it probably is of no significance to the plant in relation to radiation. For instance, the cotyledons of Geranium Ibericum rose at night to 27o above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Berendieri rose to 33o, and this would lessen radiation by 16 per cent.
There are, however, some other sources of doubt with [page 299] respect to the sleep of cotyledons. In certain cases, the cotyledons whilst young diverge during the day to only a very moderate extent, so that a small rise at night, which we know occurs with the cotyledons of many plants, would necessarily cause them to assume a vertical or nearly vertical position at night; and in this case it would be rash to infer that the movement was effected for any special purpose. On this account we hesitated long whether we should introduce several Cucurbitaceous plants into the following list; but from reasons, presently to be given, we thought that they had better be at least temporarily included. This same source of doubt applies in some few other cases; for at the commencement of our observations we did not always attend sufficiently to whether the cotyledons stood nearly horizontally in the middle of the day. With several seedlings, the cotyledons assume a highly inclined position at night during so short a period of their life, that a doubt naturally arises whether this can be of any service to the plant.