Apart from any question of hydro-mechanical transmission, it is important to distinguish two different modes of transmission of excitation. In a motile tissue contraction of a cell causes a physical deformation and stimulation of the neighbouring cell. Examples of this are furnished by the cardiac muscle of the animal, the pulvinus of _Mimosa_, and the stamen of _Berberis_. This mode of propagation may better be described as a _convection_ of excitation. The _conduction_ of excitation, as in a nerve, is a different process of transmission of protoplasmic change. The conducting tissue in this case does not itself exhibit any visible change of form. In the plant the necessary condition for transmission of excitation to a distance is that the conducting tissue should be possessed of protoplasmic continuity in a greater or less degree. This condition is fulfilled by vascular bundles. There being greater facility of transmission along the bundles than across them, the velocity in the longitudinal direction is very much greater than in the transverse. For accurate determination of velocity of transmission the testing stimulus should be quantitative and capable of repetition. Abnormal high velocity has been observed in _Mimosa_ by applying crude and drastic methods of stimulation, by a transverse cut or a burn. This is apt to give rise to a very strong hydro-dynamic disturbance, which travelling with great speed, delivers a mechanical blow on the responding pulvinus. Such hydro-dynamic transmission is not the same as physiological conduction. In the primary petiole of _Mimosa_ the highest velocity under electric stimulation I find to be about 30 mm. per second. This velocity is considerably lower than the velocity in the nerve of higher animals, but higher than in the lower animals. As an example of the latter, mention may be made of the velocity of 10 mm. per second in the nerve of _Anodon_ and 1 mm. per second in the nerve of _Eledone_. PREFERENTIAL DIRECTION OF CONDUCTION. _Experiment 33._--The conduction of excitatory impulse takes place in both directions. This can be demonstrated by taking a petiole of _Biophytum sensitivum_ or of _Averrhoa carambola_. These petioles are provided with a series of motile leaflets. Stimulation at the middle point of the petiole gives rise to two waves of excitation, one of which travels towards the central axis of the plant, and the other away from it. The centrifugal velocity is greater than the centripetal as will be seen from the following results: