I have shown ([page 198]) that a growing organ under stimulus, undergoes an incipient contraction as shown in the responsive retardation of its rate of growth; that this retardation increases with the intensity of the incident stimulus till growth becomes arrested. Above this critical intensity the induced contraction causes an actual shortening of the organ. There is no breach of continuity in the increasing contractile reaction, which at various stages appears as a retardation, an arrest of growth or a marked shortening of length of the organ.

CONTRACTILE RESPONSE OF PULVINATED AND GROWING ORGANS.

Fig. 88.—Contractile response of grow­ing organ under electric shock. Suc­ces­sive dots at inter­vals of 4″. Verti­cal lines below repre­sent inter­vals of one minute. (Mag­ni­fi­ca­tion 1,000 times.)

Experiment 94.—In order to show the striking similarity between the response of ‘sensitive’ Mimosa and that of a growing organ, I give a record (Fig. 88) obtained with a growing bud of Crinum under the stimulus of electric shock above the critical intensity. The recorder gave a mag­ni­fi­ca­tion of a thousand times. In Fig. 88, the normal growth elongation is represented as a down-curve. On the application of stimulus the normal expansion was suddenly reversed to excitatory contraction, the latent period of reaction was one second and the period of the attainment of maximum contraction (apex-time) was 4 minutes. The organ recovered its original length after a further period of seven minutes and then continued its natural growth elongation. Repetition of stimuli gave rise to successive contractile responses which are in every way similar to the mechanical responses of Mimosa pudica. The essential similarity of response of pulvinated and growing organs will be seen in the following tabular statement:

TABLE XXI.—TIME RELATIONS OF MECHANICAL RESPONSE OF PULVINATED AND GROWING ORGANS.

Specimen.Latent
period.		Apex-time.Period of
recovery.
Motile pulvinus of
 Mimosa pudica.		0.1 sec.003 secs.16 minutes.
Motile pulvinus of
 Neptunia oleracea.		0.6 "180 "60  "
Growing bud of Crinum.1.0 "240 "07  "

The contraction in growing organs under stimulus is sometimes considerable. Thus in the filamentous corona of Passiflora quadrangularis the contraction may be as much as 15 per cent. of the original length. This is not very different from the excitatory reaction of the typically sensitive stamens of the Cynereæ, which exhibits a contraction from 8 to 22 per cent.

MODIFICATION OF RESPONSE BY CONDITION OF SUB-TONICITY.