350 



PLANT RESPONSE 



Excessive pressure, however, brings on irregularity, or even 

 stoppage, of autonomous movement. 



(2) The effects of variation of tej)iperature. — I have shown 

 in the last chapter how perfectly similar are the effects of tem- 

 perature in causing variations of the ampli- 

 tude and frequency of pulsation in rhythmic 

 tissues, both vegetable and animal. As 

 regards the effect of temperature on the 

 period, the similarity is strikingly exhibited 

 in the two curves given above, showing 

 the relation between temperature and 

 period in Desmodium and in cardiac muscle. 

 It will be seen that in both (figs. 146 and 

 147) the fall of period with increase of 

 temperature is at first rapid and then slow. 

 (3) Periodic groupings of response. — 

 Another very remarkable similarity be- 

 tween the pulsations of Desviodium and of 

 cardiac muscle lies in their exhibition of 

 periodic groupings, very simple types of which, given by 

 Desmodiuui, are seen in the photographic records (figs. 148, 

 149). In fig. 146 we have an alternate waxing and waning 

 of pulsation, and a remarkably similar record, given by frog's 



Fig. 148. Simple 

 Alternation of 

 Pulsation in Des- 

 modiiiin 



Fig. 149. Periodic Groupings 

 of Pulsation in Destnodium 



Fig. 150. Simple Alternation 

 of Pulsation in Frog's 

 Heart (Penibrey and 

 Phillips) 



heart, is seen in fig. ] 50. These groupings are ot various 

 degrees of complexity, one type of such heart-beats being 

 that known as Luciani's groups, where the successive groups 

 are separated by a long pause. Now, similar groups with 



