IV] PULVINUS. LEAFLETS 43 



known to be a motile organ, and consists of soft cellular 

 tissue capable of taking up water into its cells, somewhat 

 as a pump takes it up though the action is different in 

 detail. When it becomes surcharged with water the 

 pulvinus expands, especially beneath, and forces the 

 petiole into a more erect position ; when the pulvinus 

 partially empties itself, the weight of the leaf compresses 

 it, and the petiole falls, and considerable differences of 

 angular divergence in the vertical plane may be under- 

 gone by the rigid petiole or rachis turning on its insertion 

 as on a fulcrum. 



Each leaflet may also have its own small pulvinus 

 (pulvinule) at the base of its petiolule, and may similarly 

 be raised and lowered according as the pulvinule is tense 

 and full, or flaccid and partially emptied of water. 



It will be understood that all the terms applicable to 

 the contour, margin, apex, surface, &c. of a simple leaf are 

 equally applicable to the individual lobes of a simple 

 lobed leaf, and to the separate leaflets of a compound 

 leaf; each may be of elongated, rounded, or angular 

 shapes described on pp. 22 and 28, and each may be again 

 lobed or segmented to any extent, or merely toothed, 

 serrate, hairy or glabrous as before. In the case of com- 

 pound leaves the leaflets may be again compound, though 

 such is not the case in any of the trees and shrubs here 

 to be dealt with. 



Moreover, the petiolules of leaflets may be themselves 

 provided with small stipules (stipels). 



The number of leaflets in a compound leaf is in many 

 plants constant or nearly so. In palmate leaves the 

 number is commonly five, and the leaf is often termed 

 digitate, with obvious reference to the hand, e.g. Horse- 

 chestnut (Fig. 13), but three, seven, or more may occur, 

 and a consistent terminology refers to such as tri-, quinque-, 



