SUPPLEMENT 135 



of excitation has been definitely established in heliotropism, but is only 

 probable in the case of geotropism. DARWIN (Movements of Plants) concluded 

 from experiments (similar to those already carried out by CISIELSKI) that the 

 perception of the gravitational stimulus took place, in the case of the root, in 

 the growing point at a distance of a few millimeters from the region of cur- 

 vature. When 0-05-1 mm. of the extreme apex of the root was removed, the 

 root when placed horizontally although it grew showed no geotropic cur- 

 vature. If, however, the root was first of all placed horizontally and then 

 decapitated before any curvature began to show itself, the geotropic reaction 

 took place quite normally. 



The interpretation of DARWIN'S experiment has led to an extensive con- 

 troversy (comp. ROTHERT, 1894), the net result of which is that proof of 

 apical perception is not forthcoming, for the wounding itself exerts an injurious 

 influence on the responsive powers of the plant. Hence CZAPEK (1895) attempted 

 to solve the problem by using a new method of experimentation, where wound- 

 ing was unnecessary. He allowed the root apices to grow into short capillary 

 tubes, closed at one end, and bent at right angles, while the plants were on the 

 klinostat, so that about I mm. of the end of the root formed a right angle 

 with the remainder. When these bent apices were placed horizontally so that 

 the growing region was in the rest position, a geotropic curvature made its 

 appearance. This curving did not occur, however, when the apex was in the 

 rest-lie and the growing region in the stimulus-lie. WACHTEL (1899) and 

 RICHTER (1902) have shown that the compulsory curvature induced in this 

 experiment is not without effect on the root, and that it leads to curvings 

 which CZAPEK was misled into regarding as geotropic curvatures. Although 

 CZAPEK has again put forward a plea for his method, his arguments can scarcely 

 be regarded as of any great weight, especially after CHOLODNYI'S (1906) quite 

 recent researches. By another method, which cannot be described here, but 

 which is very like in principle to that already used in discussing grass seedlings, 

 F. DARWIN (1902) has attempted to show that the seat of perception lies in 

 the apex of the root. This method also is not above criticism, and hence, 

 taking the most favourable view of the case, we can only say that, in view of 

 all these statements, apical perception is probable ; that, however, it is not 

 absolutely proved may be concluded from the fact that, on the other hand 

 (PiccARD, 1904), several methods have been employed to show that perception 

 must take place in the region of curvature. These experimental methods also 

 require reinvestigation, however, so that we need not discuss them further. 



Those authors who assume that the perceptive capacity lies in the root 

 apex are, however, by no means unanimous as to how far this perceptive 

 capacity reaches, and yet, for reasons which we shall appreciate later, this is 

 a point of great interest. At present we will only note that, according to NfeMEC 

 (1901), perception is limited to the root-cap, while CZAPEK (1906) considers it 

 as extending along the root for about 2 mm. 



In addition to the root, the shoots of seedling grasses (especially Paniceae) 

 have been shown to possess an organ to which has been ascribed the power of 

 geotropic perception. We may note at once that there can be no doubt as to 

 the perceptive powers of this organ for heliotropic stimuli ; of the perception 

 of geotropic stimuli, DARWIN'S experiments (1899 ; comp. also MASSART, 

 1902) offer only probability ; they do not exclude the possibility of the region 

 of movement being also a region where geotropic stimulation may be perceived. 

 If, following DARWIN, we lay the apices of seedlings of Paniceae horizontally, 

 curvature takes place in the growing region ; this, however, does not cease if 

 the basal region is placed in the vertical position, but continues as long as the 

 power of growth is retained. Hence the basal region finally exhibits spiral 

 windings. 



