GEOTROPISM. I 441 



the motile organ ; and (3) obviously the transference of the former to the latter. 

 The credit of having been the first to clearly distinguish between stimulus and re- 

 sponse belongs without doubt to CHARLES DARWIN (1881), although the evidence 

 he put forward in support of the fact has not stood the test of later criticism. 



Such localization and separation of perception from response have now 

 been demonstrated with certainty in the case of heliotropic stimuli (Lecture 

 XXXVI) ; whether the same is true of geotropism also is not quite so well 

 established. In spite of CZAPEK'S (1895, 1900) ingenious experiments, which we 

 cannot pause to describe here, it is very doubtful, for many reasons, whether 

 the seat of geotropic perception lies really in the root apex or only in the root- 

 cap (NEMEC, 1900, 1901) ; similarly, there is considerable room for suspecting 

 the truth of the assertion, so often made, that in the case of the seedlings of 

 certain grasses (Paniceae) it is only the extreme apex of the cotyledon that 

 is sensitive to the geotropic stimulus. [Even yet this question has not been 

 decisively answered ; new methods have been invented by PICCARD to deter- 

 mine this point (1904), but his results cannot be considered as above criticism. 

 Compare also RICHTER (1902), DARWIN (1902), and MASSART (1902).] 



Although a considerable separation between the region of perception and of 

 movement has not as yet been demonstrated in the case of the stimulus of gravity 

 there are certain recorded observations of CZAPEK (1898) available which go 

 to show that there are at least two different series of processes : those concerned 

 with the perception of the stimulus and those concerned with the reaction. 

 As in the case of all movements in response to stimulus, geotropism depends 

 on certain general or formal conditions ; there must be a certain temperature, 

 a certain nutritive supply, water and oxygen in sufficient quantity, &c., &c., 

 before any geotropic response can take place. The conditions which have 

 to be fulfilled before the stimulus can be appreciated are not the same as those 

 which govern the occurrence of the reaction, for the stimulus may be perceived 

 under circumstances where no growth or geotropic response may take place. 

 Thus at 2 C. a geotropic response 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. In this way, or by methods of a similar 

 character, we may convince ourselves of the existence of two separate phenomena, 

 phenomena of perception and phenomena of reaction. 



How comes it to have sensitivity of this kind? What is it that the 

 plant perceives when gravity affects it ? KNIGHT'S experiments leave no 

 doubt in our minds that gravity influences the plant only through mass ac- 

 celeration, which it exercises on all bodies, i. e. by weight. But it is also certain, 

 as we have seen, that the weight of the overhanging part of the plant above 

 the zone of curvature has nothing to do with it, since we can neutralize that 

 without stopping the geotropic movement. What we have to deal with is 

 an effect of weight operating in the interior of the plant, even in each individual 

 cell. Since, however, it is not infrequently the case that most of the proto- 

 plasm exhibits streaming movements it follows that it is only the quiescent 

 ectoplasm, as NOLL has pointed out (1888, 532), that can be sensitive to the 

 stimulus of gravity ; it must be able to distinguish between varying pressure 

 on different sides of the cell. Let us now assume the whole cell contents, 

 vacuoles, and streaming protoplasm to be the cause of the weight, in the vertical 

 position of the cell a lateral pressure must be exerted on the ectoplasm, but any 

 two opposite regions must be subjected to similar pressures. Let us now incline 

 the cell somewhat out of the perpendicular, then a pressure must be exerted at 

 once on a certain part of the under side somewhat greater than that on the cor- 

 responding part of the upper side. But when we remember that a considerable 



