CoGNiTic State of Energy 127 



close bond between both senses that may yet prove to be due 

 to mass-perception response, in the widest sense of the term. 

 In other words graviperception and rotation may represent 

 physiological phases of one fundamental energy and matter 

 relationsliip. Circumnutation and animal rotatory movement 

 also are preeminently the proenvironing sense, in that such 

 enable each organism to sweep round, "survey the country" 

 in circles, and finally enable it to determine on the most sat- 

 isfying line of movement. 



The hydrotactic sense is so delicately and yet powerfully 

 shoTVTi by the rhizoids of simpler plants, and by the roots of 

 those higher, that it has greatly attracted the attention of 

 botanists from the time of Sachs's publication. But, while 

 we know much as to the physiological results, we still are 

 largely ignorant as to the cell-conditions that start hydrotropic 

 movement. But probably most are agreed in regarding it as a 

 physico-chemical action, that is fairly distinct from the chemo- 

 tactic stimulation. As to its occurrence in animals, it is well 

 known that many land or amphibious genera "perceive" and 

 move toward a water supply. Most human beings also can 

 distinguish relative variations in the atmospheric moisture, 

 while some can estimate vdih wonderful exactness the hygro- 

 metric state of the atmosphere when they pass from one green 

 house to another. But our almost complete ignorance regard- 

 ing the details of the process prevents further consideration. 



The last statement is largely true for the thigmotactic or 

 mechano-i)erceptive, and for the paraheliotactic or blue-purple 

 light stimuli. It may now be safely said, however, in view 

 of the experimental results of the past twenty years, that both 

 play an important part in the life-movements of plants and 

 animals, from some of the simplest to the most complex. The 

 importance of the latter in determining the movements of 

 animals has been largely overlooked or misunderstood, even 

 where recorded, by zoologists, but has received much more 

 attention from botanists since the time of Darwin onward. 



We are still largely ignorant, however, as to the more minute 

 causes of both movements. The writer has suggested (61: 183) 



