ACTION OF GRAVITATION ON GROWTH IN LENGTH. 84 1 



It is clear that organs which are both heliotropic and geotropic, and on which, 

 since they lie obliquely to the horizon, the light falls from above or from below, are 

 subject to changes in their growth dependent both on light and on gravitation. 

 Thus, for example, the bending upwards of a branch placed horizontally on which 

 the light falls from above may be caused at the same time by positive heliotropism 

 and by negative geotropism. An erect stem, on the other hand, which turns helio- 

 tropically towards a source of light at the side and thus makes a curvature which is 

 concave below, will have a tendency to become erect in consequence of its negative 

 geotropism, and would do so if the light falling on it were removed. Stems there- 

 fore which in the evening were bent by positive heliotropism, will stand upright in 

 the morning. These considerations are evidently of the first importance in making 

 observations on the two phenomena. 



We have already seen that no clear idea has yet been obtained of the mode in 

 which light acts in influencing the growth of heliotropic organs. As litUe are we at 

 present in a condition to affirm how the acceleration or retardation of the growth of the 

 cell-walls results from the action of gravitation \ The hypotheses and considerations 

 there stated may be repeated here mutatis 7nutandis. Particular stress must be laid 

 on the fact that movements are induced in protoplasm by the action of gravitation 

 just as by the action of light. Thus Rosanoff" showed^ that the plasmodia o{ JEtha- 

 Hum sepiicum are negatively geotropic, creeping, under the influence of gravitation, 

 over steep moist walls, and turning, under the action of centrifugal force, towards 

 the centre of rotation ; they take therefore those directions which would be 

 least expected from their apparently fluid condition. The question suggests itself 

 whether there is not also protoplasm which behaves in this respect in an exactly 

 opposite manner ; and from the dependence of the growth of the cell-wall on the 

 activity and probably also on the disposition of the protoplasm in the cell, the hypo- 

 thesis must not be altogether set aside that all geotropic phenomena are in the first 

 place caused by the protoplasm taking up definite positions in the cells under the 

 [influence of gravitation, and thus accelerating or retarding the growth of the cell- walls 

 |fc>n the under sides. Since nothing is known on this subject, we must direct our 

 fattention solely to the growth of the cell-walls, leaving it undecided whether the effect 

 )f gravitation be direct or indirect. 



In order to state clearly the problem how gravitation acts on the growth of the 

 cell-waU^ we may consider as the simplest example a unicellular filament, such as we 

 find in Vaucheria, the posterior end of which developes as a positively geotropic 

 root, the anterior end as a negatively geotropic stem. Fig. 482 A may represent this, 

 assuming that the whole filament grew at first in a vertical direction either upwards or 

 downwards, but was then placed in a horizontal position, as shown by the light outr 

 lines S and W. After some time the radical end would show a downward curvature, 



* [See note on p. 837.] 



2 Rosanoff, De I'influence d'attraction terrestre sur la direction des plasmodia des Myxomycetes 

 (Memoires de la Societe imperiale des sciences de Cherbourg, vol. XIV). [According to Strasburger 

 (Wirk. des Lichts und der Warme auf Schwarmsporen, Jenaisch. Zeitschr., XII, 1878) this apparent 

 negative geotropism of the plasmodia is due simply to the fact that they tended to travel against the 

 direction of the stream of water by which they were kept moist during the experiments.] 



3 Duchartre's assertions on geotropism in his Observations sur le retournement des Champignons 

 (Compt. rend. 1870, vol. LXX. p. 781) show that he has not clearly comprehended the question. ^ 



