For purpose of simplicity, I first selected the simple type of phototropic curve, where the specimen employed was in a favourable tonic condition, and the stimulus was, from the beginning, above the minimal. Transverse conduction, which induces neutralisation or reversal into negative, was moreover absent in the specimen. I shall now take up the more complex cases: (1) where the condition of the specimen is slightly sub-tonic, (2) where the stimulus is gradually increased from the _sub-minimal_, and (3) where the specimen possesses the power of transverse conduction. EFFECT OF SUB-MINIMAL STIMULUS. It is unfortunate that the terms in general use for description of effective stimulus should be so very indefinite. A stimulus which is just sufficient to evoke excitatory _contraction_ is termed the minimal, stimulus below the threshold being tacitly regarded as ineffective. The employment of sensitive recorders has, however, enabled me to discover the important fact that stimulus below the minimal, though ineffective in inducing excitatory _contraction_, is not below the threshold of perception. The plant not merely perceives such stimulus, but also responds to it in a definite way, by _expansion_ instead of _contraction_. I shall designate the stimulus below the minimal, as the _sub-minimal_. There is a critical point, which demarcates the sub-minimal stimulus with its expansive reaction from the minimal with its responsive contraction. The _critical_ stimulus varies in different species of plants. Thus the same intensity of light which induces a retardation of growth in one species of plants will enhance the rate of growth in another. Again, the critical point will vary with the _tonic_ level of the same organ; in an optimum condition of the tissue, a relatively feeble stimulus will be sufficient to evoke excitatory contraction; the critical point is therefore low for tissues in tonic condition which may be described as _above par_. In a sub-tonic condition, on the other hand, strong and long continued stimulation will be necessary to induce the excitatory reaction. The critical point is therefore high, for tissues in a condition _below par_. Stimulus below the critical point will here induce the opposite physiological reaction, _i.e._, expansion. The physico-chemical reactions underlying these opposite physiological responses have, for convenience, been distinguished as the "A" and "D" change (pp. 143, 223). The assimilatory 'building up', A change, is associated with an increase of potential energy of the system; the dissimilatory 'break down', D change, on the other hand, is attended by a run-down of energy.