Fig. 80.—Electric response of Musa (a) Positive, (b) diphasic, (c) negative.

Experiment 85.—The records given in Fig. 80, exhibit the electric response given by vegetable tissues. On application of feeble stimulus at a distance from the responding point, the response was by gal­vano­metric positivity. Under stronger stimulus the response became diphasic, positive followed by negative. Direct stimulus induced a negative response.

VARIATION OF GROWTH UNDER INDIRECT STIMULUS.

Since the responsive reactions of growing and non-growing organs are, as we shall find later, fundamentally similar, I expected that Indirect stimulus would give rise in a growing organ to an effect which would be of opposite sign to that induced by Direct stimulus—an ac­cel­er­ation, instead of retardation of growth; that would correspond to the positive mechanical and electrical responses to Indirect stimulus given by pulvinated organs and by ordinary vegetable tissues. The account of the following typical experiment will show that my anticipations have been fully verified.

Experiment 86.—I took a growing bud of Crinum and determined the region of its growth activity; lower down a region was found where the growth had attained its maximum and may, therefore, be regarded as indifferent region. I applied two electrodes in this indifferent region about 1 cm. below the region of growth. On application of moderate electric stimulus of short duration the response was by an ac­cel­er­ation of growth which persisted for nearly a minute, after which there was a resumption of the normal rate of growth. In this particular case the interval of time between the application of stimulus and the responsive ac­cel­er­ation of growth was 12 seconds. The interval varies in different cases from one second to 20 seconds or more, depending on the intervening distance between the point of application of stimulus and the responding region of growth. I give a record (Fig. 81) obtained in a different experiment which shows in an identical specimen, (1) an ac­cel­er­ation of growth under Indirect and (2) a retardation of growth under Direct stimulus.

Fig. 81.—Effect of Indirect and Direct stimulus on growth of Crinum, taken on a moving plate. Dotted arrow shows ap­pli­ca­tion of In­direct stimu­lus with con­se­quent ac­cel­er­ation of growth. Direct ap­pli­ca­tion of stimu­lus at the second arrow induces con­trac­tion and sub­se­quent re­tard­ation of rate of growth. Suc­ces­sive dots are inter­vals of 5″. (Mag­ni­fi­ca­tion 2,000 times).