I 3 4 PLANT PHYSIOLOGY 



certainly, in a short time, inverted roots curve downwards and inverted shoots 

 upwards. 



440, 1. 23 P. 441, 1. 38, for Any attempt . . . affects it ? read Since a geo- 

 tropic stimulus reaction follows in an obliquely placed axis on the klinostat, 

 it must be concluded that the stimulus is perceived also in an axis laid quite 

 horizontally, but that this perception cannot lead to curvature because it is 

 the same on all sides, i. e. the klinostat does not prevent stimulation, but only 

 the visible reaction. This conclusion could be arrived at, before the publica- 

 tion of FITTING'S researches, only from the behaviour of the nodes of grasses. 

 These structures when laid horizontally were induced not only to curve but 

 to start growing afresh. When they were placed on a klinostat, they began 

 to grow there also (ELFVING, 1884), but the growth was naturally uniform on 

 all sides. 



From the work which has hitherto been carried out on this subject it 

 may be deduced with perfect certainty that between the application of the 

 gravitational stimulus on the one side and the geotropic curvature on the 

 other, a whole series of processes takes place as to whose nature we are cer- 

 tainly at present very much in the dark. It is obvious that in the first instance 

 gravity must have a certain purely physical effect on the plant. Gravity 

 must exert a pressure due to weight on some part of the plant or of the cell, 

 but to this point we shall return later. The effect of this pressure is to cause 

 a change of unknown character hi the protoplasm, possibly of the nature of 

 a compression. Although it has not as yet been proved that this change is 

 dependent on the persistence for a certain time of the action of gravity, still 

 we are forced to believe that it is so ; we must assume that gravity must act 

 for a certain time in order to induce this change. After the primary purely 

 physical action of gravity (assuming it to be of sufficient intensity and duration), 

 there follows the physiological result spoken of as ' excitation '. When excita- 

 tion has begun we say that the plant has ' perceived ' the gravitational stimulus. 

 While, however, the physical change must pass off as rapidly as it arose after 

 the removal of its determining cause, the excitation lasts longer it persists 

 even for as long as twelve times the period of stimulation. If now, after a 

 period of rest, but before the excitation has quite faded away, we apply a second 

 stimulus of like duration and intensity to the first, the same physical change 

 will be produced, but the excitation which follows from it will be added to 

 what remains of the first excitation. In this way a degree of excitation is 

 gradually reached by continuous summation, which will lead in the end to 

 movement ; the excitation has now overstepped the liminal value needed to 

 induce response. 



That excitation and reaction are two very different processes may be 

 deduced not only from the fact that there may be excitation without reaction, 

 but also from the fact that the two processes depend on external conditions 

 in entirely distinct ways. Excitation often still takes place when growth 

 geotropic movement is no longer possible. Thus at 2 C. a geotropic re- 

 sponse may be induced after a sufficiently long exposure to the stimulus of 

 gravity, but the movement is carried out only when the plant is exposed to 

 a higher temperature. Again, the stimulus of gravity may be appreciated in 

 an atmosphere free from oxygen, but that gas is essential for the carrying out 

 of the movement. The processes which intervene between excitation and 

 reaction are, generally speaking, unknown. In certain cases, however, we can 

 convince ourselves of their existence with ease, e.g. when the reaction takes 

 place in some other situation than perception or excitation. Obviously in 

 such a case conduction must occur between the two regions. Conduction of 

 this kind usually spoken of as conduction of stimulus, but more accurately, 



