i68 THE STORY OF PLANT LIFE 



secreted in five glands at the base of the inner 

 stamens to which honey-guides on the petals con- 

 verge. The claws of the petals are hairy, to protect 

 the honey from rain and creeping insects. In the 

 other small-flowered types the stigma is ripe first. 



The seeds or carpels are jerked to a distance by a 

 catapult mechanism. 



The group includes numerous showy plants which 

 are familiar garden flowers. Many are aromatic and 

 astringent, and yield volatile oils. 



Wood Sorrel {Oxalis Acetosella). 



Some plants have the faculty of sleeping, and 

 exhibit sleep movements, or nyctitropic movements, as 

 they are called. These movements are related to 

 the variation in the intensity of light due to the 

 alternation of day and night. At night the leaves, 

 fully expanded in the day, close up, and the edges are 

 turned upwards, and in the case of compound leaves 

 the leaflets close up. 



This is due to a change in the turgidity of the 

 parenchymatous cells. The effect of the sleep move- 

 ments is to prevent loss of water by transpiration, 

 and to protect the plants from cold, which checks 

 absorption. 



In the Wood Sorrel there are four types of move- 

 ment. The whole leaf moves, changes its position, 

 the chlorophyll granules in the leaf change their 

 position, and the granules change their form. A 



