266 MOTIONS CAUSED BY LIGHT AND MOISTURE. 



closing together at night, and unfolding in the morning. There 

 are a few species, however, which unfold at night and close 

 during the day. There are some, too, which close during the 

 day, when the sky is overcast and a storm is threatened. These 

 changes seem almost entirely dependent upon the degree of light, 

 to which the plant is exposed ; for they may be made to take 

 place at the contrary periods, by keeping the plants in a dark- 

 ened room during the day, and placing them at night in strong 

 lamplight. It is usually some little time, however, before they 

 become accustomed to the change ; and their movements are at 

 first irregular. The mode in which light produces these move- 

 ments has not yet been ascertained ; but it can scarcely be 

 doubted, that it is by its influence on the exhalation of fluid from 

 the soft tissues, on one side of the bending part more than on 

 the other. Supposing that the part were otherwise bent, the 

 influence of light upon the cells of the convex side would cause 

 them to contract, and thus straighten it, a change which we 

 shall presently see to be elsewhere effected by another cause 

 acting in like manner. "Whilst, if the part were straight in the 

 dark, so that the leaves were erect, and the flowers expanded, 

 the influence of light, acting more on one side than on the other, 

 would cause it to bend towards that side, by causing the tissue 

 to contract. 



418. The influence of water, or of varying degrees of mois- 

 ture in the atmosphere, seems often to produce movements in 

 the living plant, as well as in dead portions of its tissues. It is 

 in this way that the closure and unclosure of the Rose of Jericho, 

 and the Lycopodium of Peru, are occasioned, the one by 

 drought, the other by the contact of fluid. This is easily ac- 

 counted for, by supposing that the cells on one side are larger, 

 and have thinner walls than those on the other ; and these will, 

 therefore, be most easily distended when placed in water, and 

 will soonest lose their fluid in drying. The beards of the Gera- 

 nium and Wild Oat curl up in dry weather and straighten in 

 damp ; those of some other plants perform the contrary move- 

 ment. Such parts of plants are often used in the construction of 

 hygrometers, to indicate the amount of dryness in the atmosphere, 

 to detect dampness in beds, &c 



