GEOTROPISM. I 



443 



connexion between perception and response. Further, we do not know enough 

 as to the geotropic response. Noll appears to assume that the cells of the 

 concave side are just as active in the reaction as are those of the convex side, 

 but the retardation of growth in them can be also passive and due merely to the 

 accelerated growth of the convex side (p. 435). Noll's hypothesis brings up 

 quite special difficulties, however (compare Pfeffer, 1893 a), if the perception 

 really takes place elsewhere than does the movement, for according to certain 

 observations of Czapek (1898), it is quite certain that the perception of the 

 under region of a horizontally laid root apex cannot be different from that of 

 the upper side, and that it is only transmitted to the under side of this motile 

 zone. On the contrary we must assume equal perception in all the cells of the 

 apex, and yet the cells of the motile region react quite differently. 



Many of the criticisms we have advanced against Noll's hypothesis 

 are apphcable also to Haberlaxdt's (1900) and Nemec's (1900) views, simul- 

 taneously and independently promulgated. These authors also employ a 

 statocyst hypothesis. The sensitive plasma is the ectoplasm of the entire cell, 

 the statoliths are relatively heavy bodies, such as crystals and starch gi^ains. 

 Starch grains, which respond quickly to the influence of gravity and press 

 against a different region of the ectoplasm when the plant is in the upright 

 than when it is in an inclined or horizontal position, are found in many plants, 

 regularly in the starch sheath of the stem, and in the root in a central group 

 of cells of the rootcap. Fig. 137 shows them in the apex 

 of a cotyledon of a grass. The cells which contain motile 

 starch grains of this kind are conceived by the authors above 

 mentioned to be the sense organs for the geotropic stimulus. 

 This hypothesis has this great advantage that it can be 

 examined into with the aid of a microscope. It has called 

 forth a whole series of most interesting observations (Haber- 

 LANDT, 1903 ; Darwin, 1903), into the discussion of which 

 we have unfortunately no time to enter. But difficulties of 

 various kinds meet us here also ( Josx, 1902). We will confine 

 ourselves to showing that evidence is entirely wanting tending 

 to prove that only cells with movable starch grains are able 

 to appreciate the stimulus of gravity. It will be sufficient 

 to draw attention to the fact that there are plenty of plants F'g- j37- Apex of 



V . I 1 •! •. x-i J. ■ •• 1 J, 1 • 1 J. • the cotyledon ot Pani- 



which exhibit the geotropic reaction, but which contam no cum mhiacenm. After 

 starch, just as, conversely, there are cells with movable starch N^iiEc (1901). 

 gi"ains which give no geotropic response, although capable of 

 gi'owth. [The starch-statolith theory of geotropism has given rise to a flood 

 of publications, and, although it has certainly found more supporters than 

 opponents, we cannot as yet accept it as proved. Since, in the course of the 

 controversy, Haberlandt has admitted that a perception of gravity may occur 

 without any change of position of the starch grains, he has placed the theory 

 in an unassailable position, but at the same time rendered it impossible of proof. 

 Further details will be found in the literature, of which we quote only the most 

 recent and most important (Haberlandt, 1905 ; Nemec, 1905 ; Darwin, 

 1904 ; Noll, 1905 ; Fitting, 1905).] 



Although a study of the newer investigations, which have aimed at analysing 

 the process of stimulation in geotropism, has led us to no conclusive result, we 

 must yet admit that our knowledge has been essentially widened on the subject, 

 for w^e have had convincing evidence that the whole process is a most compli- 

 cated one and consists of many inter-related phases. In the first place we 

 have the purely mechanical phase : gravity acts through weight affecting an 

 unknown part of the cell — probably the sensitive plasma. It is not at all 

 improbable that pressure on the plasma induces sensation through the weight 



