. A textbook of botany for colleges and universities ... Botany. 452 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 temately com- pound the petiolules or stalks of the leaflets are motor organs. Thus Mimosa has primary, second- ary, and tertiary motor organs (fi
. A textbook of botany for colleges and universities ... Botany. 452 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 temately 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- ch)Tiia 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- - Leaf of Mimosa in open and closed proach one another SO closely positions. — From Part III. ^u ^ ^u • 1 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.' 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. ' This is the reason for a technical name applied to the motor organ, the Fig
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