GEOTROPISM. I 



437 



be supported by a force equal in amount and yet that the geotropic curva- 

 ture was still maintained, and in 1829 Pinot found that the root apex 

 was able, when bending downwards, to force its way into mercury, and could 

 overcome a very considerable opposing pressure. Hofmeister was, however, 

 unacquainted with these older observations, and Frank (1868) was the first 

 to replace the view of passive sinking of the root apex held by Hofmeister 

 by the correct interpretation, when he said that in geotropism we were dealing 

 with a special evolution of energy which was released in the interior of the 

 organ by gravity. Subsequently the effect of gravity as a releasing force was 

 fully discussed by Pfeffer (1875, 1893 a) and by Sachs. It ought also to be 

 mentioned that, in 1824, Dutrochet spoke of gravity as a releasing agent, 

 although, later, he enunciated another view. 



As the downward curving of the root may be accompanied by a recog- 

 nizable expenditure of energy, so also negative geotropism is the result 

 of a similar expenditiure, because a weight has to be lifted, and because the force 

 acts on a very much larger lever in the shoot than in the root. Pfeffer (1893 b) 

 has recently studied the work done in the nodes of grasses subjected to geotropic 

 curvature, and Meischke (1899) has examined other plants with a similar 

 end in view. It has been proved that the energy required for carrying out 

 the work accomplished reaches values such as we might expect to obtain on 

 the principle (p. 422) that geotropism is a growth phenomenon. It has been 

 shown more particularly that the energy expended in making a stem of a grass 

 stand erect is about that required for the purpose, but that, as a general 

 rule, a large surplus of energy is developed in other geotropic curvatures which 

 permits of the straightening of the shoot after pronounced over-curving of the 

 apex. Although we cannot go into details here it will be sufficient for us to know 

 that the work done during the curving bears no ratio to the energy provided 

 by gravity. The energy required to produce the movement is supplied by the 

 growing parts of the plant itself, gravity acts merely as a releasing force. 



If gravity acted only through the weight of the moved organs the nature 

 and amount of the resulting curvature might be determined according to 

 mechanical principles, but if it acts as a stimulus we must first of all de- 

 termine experimentally how far the curvature depends on the duration, in- 

 tensity, and direction of the force. Even in rapidly reacting organs there is 

 always an interval of about one to one and a half hours, before the horizontally 

 placed organ shows a noticeable curvature, and this latent period may in other 

 cases be extended to several hours. It is by no means essential that the plant 

 should be stimulated continuously until the reaction begins — it is quite sufficient 

 if the stimulus be applied for a shorter period. As Czapek (1898) has shown, 

 the sporangiophores of Phycomyces, the hypocotyl of Beta, the first sheathing 

 leaf in seedlings of Avena sativa and Phalaris canariensis, after exposure 

 to the geotropic stimulus for only fifteen minutes, exhibit later a curving, even 

 if meanwhile they be placed in a vertical position, or, better still, rotated on a 

 klinostat. An ajter-e'ffect following on a geotropic stimulus makes itself apparent 

 not only if the organ during the curving is gradually placed in its normal rest 

 position but also if the stimulus be removed long before any visible reaction 

 occurs. The minimum time during which the geotropic stimulus must be applied 

 in order that a bending may take place as an after-effect, we term the latent period. 

 It has in no case been found to be less than fifteen minutes, and amounts to 

 twenty minutes in the radicles of Zea, Pisiim, Lupinus, and Cucurbita, to fifty 

 minutes in the epicotyl of Pkaseolus, and to several hours in other structures. 

 [According to Fitting's (1905) researches Czapek's numbers are not to be relied 

 on ; the latent period may often amount to only 6-7 minutes, and, according to 

 Moisescu (1905), it may be even less than that. There is no doubt that the latter 

 author's employment of the microscope for the determination of the commence- 



