45 2 



PHYSIOLOGY 



they are also found in a considerable number of families allied to the 

 Berberidaceae and Scrophulariaceae. 



Structure. The leaves of Leguminosae are usually much branched, 

 and the primary motor organ, when present, is located at the base of 

 the main petiole. In many cases there are also motor organs (secondary) 

 at the origin of the secondary petioles, and if the leaf is ternately com- 

 pound the petiolules or stalks of the leaflets are motor organs. Thus 



Mimosa has primary, second- 

 ary, and tertiary motor organs 

 (fig. 683) ; but the red and sweet 

 clovers have only one set, the 

 stalks of the leaflets. The 

 motor organ consists of all or 

 a portion of the petiole or peti- 

 olule, modified by changes in 

 the position of the vascular 

 bundles, and by an excessive 

 development of the paren- 

 chyma of the cortex. Through 

 the greater part of the leaf 

 stalk the vascular bundles lie 

 at some distance from the 

 center, surrounding a distinct 

 pith, and within a cortex of 

 moderate thickness. In the 

 motor organ, however, they ap- 



FIG. 683. Leaf of Mimosa in open and closed proach one another so closely 

 positions. From PART III. 



that there is scarcely any cen- 

 tral pith, and they form a shaft, elliptical or kidney-shaped in section. 

 Outside, the cortex is correspondingly larger, and its cells are usually 

 somewhat different from the rest. As a whole the motor organ is some- 

 times thicker than the other part of the petiole, but it is quite as likely 

 to be smaller ; in all cases, however, the relative increase of the cortex 

 in cross section gives the impression of a cushion of parenchyma. 1 

 In this region the cells are rather regular in form, approximately 

 cylindric, and with smaller intercellular spaces than in the nutritive 

 regions. Intercellular spaces are present, however, at the junction of 

 three or more cells. 



1 This is the reason for a technical name applied to the motor organ, the pulvinus. 





