Page:Researches on Irritability of Plants.djvu/99

76 often be obtained in this way after a preliminary period of variation.

The periodic variation seen in the above cases sometimes finds still more complex expression. This is the case where waning and waxing occur in series instead of simple alternation. That is to say, response may undergo a continuous diminution in a sequence of three or more, to be followed by a corresponding sequence in which the amplitudes wax larger and larger, such serial alternations being repeated.

We have seen the responses that characterise highly excitable specimens, in which there is an exhibition of growing fatigue. Taking a specimen in the contrasted condition of more or less sub-tonicity, we obtain an equally characteristic effect, which is the antithesis as it were of that which we have been considering. In this, successive responses undergo a gradual enhancement, or what is known in muscle-response—with which it is exactly parallel—as a staircase increase (figs. 37, 38). After attaining a maximum excitability, under successive stimulations, there generally ensues a fatigue-decline.

Before entering on a detailed description of this particular response it would be well to discuss certain phenomena characteristic of a relatively a-tonic condition of the tissue. In a specimen in the normal condition there is a certain amount of tonicity, accompanied by a moderate degree of contraction. When deprived of the invigorating influences of favourable external stimuli the plant becomes sub-tonic, such relative a-tonicity being characterised by relaxation or the absence of normal tonic contraction. Under the action of successive stimuli the tonic condition of the specimen will be improved. The loss of tone, with its consequent relaxation, will gradually give place to a better tone with increasing tonic contraction. Or the