The New Student's Reference Work/Motor Organ
Mo′tor Or′gan (in plants), a term applied to a portion of the leafstalk (petiole) which is sensitive to certain stimuli (see Irritability) and has a special structure enabling it to curve easily. Motor organs are most perfect in the bean and oxalis families, but exist also in some spurges (Euphorbia), the common mallow (Malva rotundifolia) and the velvet leaf (Abutilon Avicennæ). To the eye the motor organ usually is of a different color from the rest of the leafstalk, and either larger or smaller. If the leafstalk is long, the motor organ will be at the base; if short, the whole stalk may be a motor organ. In compound leaves there may be motor organs at main and secondary (and even at tertiary) petioles. In contrast with the rest of the petiole the woody parts of the motor organs are gathered near the center, and the whole of the surrounding tissue is made of thin-walled cells. When their turgor (which see) increases on one side and decreases on the other, the motor organ becomes curved, carrying the leaves into a new position. See Movements.