Page:Life Movements in Plants.djvu/281

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RESPONSE IN GROWING AND NON-GROWING ORGANS
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Under condition of sub-tonicity the autonomous activity of leaflet of Desmodium gyrans and of growing organs comes to a stop. The arrested activity in both is revived by the application of stimulus. Active pulsation in Desmodium, and active growth in growing organs are, however, retarded or arrested by stimulus.

The contractile effect of stimulus on pulsation of leaflets of Desmodium gyrans is seen by the reduction of the diastolic limit of its pulsations; to this corresponds the incipient contraction and retardation of rate of growth in a growing organ. The effect of warmth is antagonistic to that of stimulus. The expansive effect of rise of temperature is seen in Desmodium by the reduction of the systolic limit of its pulsation; in growth it is exhibited by an acceleration of the rate of growth.

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

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

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