CHEMISTBT AND PHTSIOS OF PLANTS. 93 



tion of movement, known as geotropism; and, lastly, we may 

 regard, the irritableness of certain plants, as, for example, 

 the sensitive- plant, as a high specialization of the general 

 power of movement possessed at some time or other by all 

 plants and all parts of plants, 



191, In regard to the sleep of plants, observation has 

 shown that at night the cotyledons (first leaves of the seed- 

 ling) of many plants take a different position from that 

 which they have during the day. In the cabbage and 

 radish, for example, the cotyledons stand during the day 

 almost at right angles to the stem, but at night they rise 

 and are parallel to one another. Seedlings of parsley, cel- 

 ery, tomato, and four-o'clock behave in a similar manner. 

 In some cases the cotyledons instead of rising, at night, 

 bend abruptly downwards. This happens with seedlings 

 of certain kinds of sorrel (Oxalis), although curiously in 

 other species of the same genus the cotyledons rise. 



192. The leaves of many (if not all) plants assume a po- 

 sition at night more or less different from that which they 

 have during the day. In the common purslane the leaves 

 at night bend upwards in such a manner as to lie more 

 nearly parallel with the stem. In wood-sorrel (Oxalis) the 

 leaflets bend abruptly downward and closely surround the 

 common leaf -stalk. In clover, on the contrary, the leaflets 

 bend upwards, afterwards folding over to one side. In 

 beans the leaflets sink down somewhat after the manner of 

 the wood-sorrel. In some cassias and the sensitive-plants 

 the nocturnal position differs remarkably from that of the 

 day; not only are the leaflets folded, but the leaf -stalks 

 change their position, in some cases rising and in others 

 becoming sharply depressed. Even some conifers have 

 been observed to show a well-marked sleeping state at night. 



