‘ CHEMISTRY AND PHYSICS 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 tne 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 
leafiets 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. 
