CHAP. III.] PROTECTIVE ARRANGEMENTS. 103 



non- development of the naked portions of the midrib 

 between the pairs of leaflets in the pinnate type. 



When a species has reached the stage of producing 

 compound leaves, the next step necessary for the folding 

 up of the leaf is the evolution of a certain amount of 

 irritability or responsion to external agents, as light and 

 heat, by the protoplasm. The closing of the leaflets of 

 compound leaves is usually influenced by the relative 



Fig. 29. A digitately compound leaf of the horse-chestnut (/Escu- 

 Ills hippocastanum). The young leaves are sensitive and close up at 

 night. 



amount of light, providing the temperature is sufficiently 

 high. The closing and opening depends entirely on 

 changes taking place in the joints or articulations of the 

 leaflets and of the joint that connects the entire leaf to 

 the branch. These changes again are due to the move- 

 ment of water from the cells of one side of the joint 

 to the other, the movement of the water being set up 

 by the influence exercised by external agents on the 



