DIV. ii PHYSIOLOGY 347 



cases (moss-rhizoids) geo-perception is still possible after the disappearance of 

 starch. If, however, the investigations of ZOLLIKOFEB, are confirmed, according 

 to which, after disappearance of the starch, the power of geotropic reaction is 

 lost, while growth and phototropic reactions continue, the statolith-hypothesis 

 would have received the long-sought support. 



As a rule we can only infer the geotropic irritability of an organ 

 from the curvatures that take place, but in some cases it can be 

 done independently of this reaction. Thus, for example, in some 

 grass seedlings (Paniceae) that have a well-developed iuternode be- 

 neath the sheathing leaf, the latter becomes full grown and no longer 

 capable of curvature ; it is, however, still geotropically sensitive, since 

 on the sheath being exposed to the one-sided action of gravity the 

 internode below, which is not itself sensitive to the stimulus, becomes 

 curved. The geolropic stimulus must have been conducted from the 

 sheathing leaf to the internode. In other grass seedlings (Poaeoideae) 

 it has been observed that the tip of the sheath is much more sensitive 

 to the geotropic stimulus than the zone of maximal growth, and a 

 similar diminution of the sensibility on passing backwards from the 

 tip holds for roots. It is possible with special apparatus to stimulate 

 geotropically in opposite directions the apex and growing zone of 

 such objects by centrifugal force, and to show that the curvature 

 of the growing zone is then determined by the stimulated tip. There 

 is thus a conduction of the stimulus in the basal direction which 

 overcomes the direct stimulation of the growing zone. In such cases 

 a clear separation of three processes is evident, the reception of the 

 stimulus (perception), the conduction of the stimulus and the reaction. 

 An organ may be perceptive without being able to react or conversely. 

 We are justified in assuming that these three parts of the process 

 must be distinguished in cases where they are not so evident. 



It can be inferred from these experiments that the degree of 

 geotropic curvature and the rapidity with which it is produced in 

 no way measures the amount of the stimulus, since they are largely 

 dependent on the capacity for growth. The degree of geotropic 

 stimulation depends both on the specific receptivity of the stimulated 

 organ and on the amount of stimulus which it has received. For 

 any given organ it is directly proportional to the amount of stimulus. 

 By this is understood the product of the intensity of the stimulus 

 and the duration of its action. Thus, it is the same so far as result 

 is concerned, whether a high centrifugal force for a short time or a 

 less force for a correspondingly longer time be employed. 



This law ( 104 ) holds good only within certain limits. It has been 

 shown that an orthotropous organ, when laid horizontally under 

 constant external conditions, begins to curve after a definite time. 

 The period from the commencement of stimulation to the commence- 

 ment of the reaction is termed the REACTION-TIME. To obtain a 

 geotropic reaction, however, it is not necessary to stimulate an organ 



