THE CARDINAL POINTS FOR GROWTH 75 



C. Correlative stimulation, in which functions not directly affected are 

 indirectly modified. This occurs very commonly, although 

 growth is not always perceptibly influenced. 



Lastly, in all the above types we are concerned with 



(i) Homogeneous or diffuse stimuli ; (2) unilateral or directive stimuli ; 

 (3) transitory stimuli ; (4) stationary or permanent stimuli. 



Transitory stimuli usually produce no permanent formative changes. In 

 the case of wound-reactions we are dealing partly with a permanent stimulus 

 owing to the persistent after-effect. Similar classifications apply to internal stimuli, 

 which are, however, more difficult to determine. The difficulty of classification is 

 rendered still greater by the fact that frequently the same agent may produce 

 several effects, and that the nature of these may differ according to the other 

 conditions. Thus, in the presence of particular food, light becomes non-essential 

 for certain facultatively heterotrophic algae, and oxygen for certain facultative 

 anaerobes. Further, a change in the external conditions causes Vaucheria to 

 produce sexual organs instead of swarm-spores, and vice versa. Moreover, no 

 definite distinction can be made between normal and abnormal growth. Indeed, 

 any one who had only seen the aquatic form of an amphibious plant or the 

 yeast form of Mucor would regard the aerial forms as abnormal growths due to 

 the unusual conditions. Pathological growths, whatever their origin may be, 

 always result from definite causes, and in many cases their study, although 

 really a subject apart, affords important evidence as to the normal progress of 

 various functions. 



Historical. Dutrochet 1 was the first to clearly distinguish between the 

 essential formal conditions for growth and the reactions due to external 

 agencies or arising from internal stimuli. These relationships were further 

 discussed by Sachs, and a deeper view into the nature and mode of such 

 responses has been given by Pfeffer 2 . 



PART II 



THE INFLUENCE OF TEMPERATURE 

 SECTION 22. The Cardinal Points for Growth. 



THE dependence of growth on temperature is well shown by the 

 partial or complete arrest of growth during cold spring days, and its 

 resumption or increased rapidity when the temperature rises sufficiently. 

 Similar observations reveal the dissimilar requirements of different plants 



1 Dutrochet, Rech. anat. et physiol. s. 1. structure interne d. animaux et ve"ge"taux, 1824, 

 ). 8-162. 



2 Pfeffer, Die Reizbarkeit d. Pflanzen, 1893, PP- I2 > l8 (repr. from Verhandl. d. Ges. deutsch. 

 Naturf. u. Aerzte, 1893); Sachs, Flora, 1863, p. 449. Sachs termed all movements resulting from 

 stimuli paratonic. 



