THE relation of the vegetal organism to its environment has demanded a much more generalized type of sensory action than that of the animal, Thus but few species of plants have developed special perceptive organs. The sensory functions are exercised by extended regions of the body, yet the delicacy of appreciation of differences in the intensities of external forces is not surpassed by that of the animal. Thus no plant has sensory organs for the reception of light-stimuli, yet, as a matter of regulation of their main function of food-building, leaves react to differences in intensity far beyond the range of the unaided human eye. Special tactile organs are differentiated in tendrils and in certain '^carnivorous' species and 'sensitive' plants,
|Fig. 2. Longitudinal Section through a Single Papilla of an Epidermal Cell of the Columna of Stylidium graminifolium which is Sensitive to Contact.||Fig. 3. Epidermal Cell of the Perceptive Layer of Tendril of Entada scandens.|
in which members are adapted to a narrow and unusual non-typical purpose. Here also great delicacy and accuracy is obtained, and the contact or weight of a body inappreciable to the sense of touch of any known higher animal may act as a stimulus. This refinement of reaction in undifferentiated tissues is quite remarkable. As a further instance it may be cited that leaves of certain seedlings are capable of appreciating an intensity of light equal to .00033 of a standard candle.