684 PLANT RESPONSE 



indicated by the periodic erection and depression of all the 

 leaves synchronously, which will thus act, so to speak, as 

 signal-flags. 



Other modes of exhibition of diurnal periodicity of 

 hydrostatic tension. — There are also other modes besides 

 that of mechanical response of the leaves by which this 

 diurnal periodicity of tension can be detected ; and Millardet 

 has shown that the maximum tension in Mimosa occurs at 

 dawn, when the primary petiole is in its most erect position. 

 The minimum tension, again, occurs in the evening, when the 

 leaf occupies its most depressed position. Kraus, further, has 

 found the organs of the plant diminish in bulk from morning 

 till afternoon, and that the reverse process takes place from 

 afternoon till morning. Growth itself, again, is well known 

 to exhibit a diurnal periodicity. It is interesting, however, 

 to realise that this is simply the mode by which a radial 

 organ exhibits, in a form of longitudinal response, what was 

 otherwise exhibited in Mimosa as lateral response. The 

 periodic variation of tension induced by the diurnal period 

 has been seen to manifest itself in Mimosa in two alternate 

 movements, positive and negative. Similarly we shall find, 

 in growth- response, positive and negative variations above 

 and below the normal or average rate. If we take a 

 balanced record of growth, representing its average rate, a 

 downward line will indicate retardation or negative response, 

 while an upward line will indicate an acceleration of growth, 

 or positive response ; and in thus recording variations of the 

 rate of growth, by the balanced method, for a period of 

 twenty-four hours, we obtain, as we have seen (fig. 191), 

 a curve which closely resembles that of the nyctitropic 

 movement. 



Forced vibration and its periodic after-effects. — We 

 have thus traced out the two alternating impulses — the direct 

 effect of external stimulus and its direct after-effect, and the 

 internal stimulus with the negative after-effect -which induce 

 the forced vibration of the responding organ. The periodic 

 effects of protoplasmic contraction and expansion, induced 



