252 GROWTH 



and out of the vacuoles of the cells, and the turgid condition of 

 the cells becomes altered in consequence. 



The flowers of scarlet pimpernel and other plants close in the 

 daytime if the weather is dull and the air damp. By closing 

 during unfavourable weather the stamens and other reproductive 

 parts are protected against possible injury from rain and other 

 causes, and by opening on warm days the plant secures a better 

 chance of cross-pollination, for only at such times are insect 

 visitors abundant. 



The leaflets of the compound leaves of the clovers, medicks 

 and other Leguminosse, as well as those of wood-sorrel and other 

 plants, fold together or change their position in a characteristic 

 manner at night and open out again next morning. Movements 

 of this kind are termed nyctitropic or sleep-movements, and are 

 effected by the plants, in response to the stimulus of varying 

 temperature and altered illumination occurring during the 

 changes from day to night. Frequently the edges of the leaves 

 and leaflets are turned upwards at night, or the whole leaf 

 droops or is folded in such a way that the leaf-area presented 

 to the sky is much diminished, and loss of heat by radiation is 

 consequently reduced. By taking up such positions at night 

 the leaves are considerably protected from being injured by cold. 



Ex. 153. — Examine the ' day ' and ' night positions ' of the leaves of clover, 

 medicks and runner beans. 



In the daytime cover up a white clover plant with a bowl or basin, and 

 after two hours compare the induced ' night position ' of the leaflets of the 

 darkened plant with the day position of the leaflets on a neighbouring ex- 

 posed plant. 



Ex. 154 — Compare the day and night positions of the flowers of wild 

 carrot, Geranium Robertianum, and wild pansy. 



Pluck two or three full-grown crocus and tulip flowers when closed in the 

 morning of a dull day ; place their stalks in water and convey them to a 

 warm room. Notice how soon they open : after opening, stand them in a 

 cool place and observe how soon they close. 



Ex. 155. — On a, bright day pluck some well-opened heads of daisies and 

 dandelions : place the stalks in water and then transfer them to a dark 



