Page:Life Movements in Plants.djvu/281

Rh Under condition of sub-tonicitv the autonomous activity of leaflet of Dssmodiuin gyrans and of growing organs comes to a stop. The arrested activity in both is revived by the application of stimulus. Active pulsation in Des- modiian, and active growth in growing organs are, however, retarded or arrested by stimulus.

The contractile effect of stimulus on pulsation of leaflets of Dfi^mttdium gurans is seen by the reduction of the diastolic limit oc its pulsations ; to this corresponds the incipient contraction and retarilatiou of rate of growth in a growing organ. The eilect of warmth is antagonistic to that of stimulus. The expansive efi'ect of rise of tempera- ture is seen in Desmndiitin l)y the reduction of the systolic limit of its pulsation ; in growth it is exhibited by an acceleration of the rate of giowtli.

All stimuli which induce an cvicitatory contraction and fall of the leaf of Mimosa also induce incipient contrac- tion and retardation of rate of growth in a growing organ.

Excitatory' effects of differerit rays of light on motile and growing organs are similarly discriminative. Ultra- violet light exerts the most intense reaction which reaches a minlinum towards the less refrangible red end of the spectrum. Beyond this, the infra-red or thermal rays bacome suddenly effective in inducing excitatory movement and retardation of growth.