426 



PLANT RESPONSE 



Fig. 172. Initiation of 

 Erectile Response in leaf 

 by Supply of Water to 

 partially Drought-rigored 

 Mimosa 



(3) Renewal of growth by positive turgidity-varia- 

 tion. — We have seen that in Desmodium in a state of standstill, 

 increased internal hydrostatic pressure renewed the rhythmic 

 activity. It was also stated in the last 

 chapter, that growth is a responsive 

 expression of the positive turgidity- 

 variation. We have seen further that 

 the mechanical expression of the 

 positive turgidity-variation in a dorsi- 

 ventral organ takes the form of 

 erectile response. Thus this erectile 

 response and growth-elongation are 

 to be regarded as two different forms 

 of expression of the same internal 

 activity. 



If we take for example a plant of Mimosa which is under- 

 turgid, for want of sufficient supply of water, but not to the 

 extent of drought-rigor, the leaves are found to assume a 



certain horizontal position, corre- 

 sponding to the degree of tur- 

 gidity. If we now supply the 

 plant with water, poured on at 

 the roots, the consequent sudden 

 increase of suctional pumping 

 activity is seen in the positiveerec- 

 tile response of the leaf (fig. 172). 

 Similarly, when an ordinary 

 plant, under the same circum- 

 stances, has its growth brought 

 to a standstill, the growth-elonga- 

 tion is found to be renewed on 

 the application of a fresh supply 

 of water. This experiment was 

 carried out on a seedling of 

 Cucurbita 12 cm. in height, growing in a small pot, which 

 had come to growth-standstill for reasons described. Two cc. 

 of water was supplied to the dry soil about the roots, and 



Fig. 173. Initiation of Growth- 

 pulsation by Small Supply of 

 Water to Drought-rigored Seed- 

 ling of Ctuurbita 



The first thick dot represents 

 application of water, which 

 induces growth for three minutes 

 only. A second application at 

 the second dot renews it for the 

 second time. 



