760 



MECHANICAL LAWS OF GROWTH. 



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 mutandis. 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 oi ^tha- 

 lium 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 



Fig. 452. — Diagram for illustrating geotropic upward and downward curvature. 



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

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

 attention solely to the growth of the cell-walls, leaving it undecided whether the efTect 

 of gravitation be direct or indirect. 



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

 cell-wall^, we may consider as the simplest example a unicellular tube, 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. 452 ^ may represent this, 

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

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

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

 like W\ the part 6" on the contrary the upward curvature, as S\ It is self-evident 

 that each of these curvatures can only result from the growth, equal on all sides 

 when the organ is erect, having now become unequal on the upper and under sides, 

 the convex growing in both cases more quickly than the concave side. 



* Rosanoff, De Tinfluence d'attraction terrestre sur la direction des plasmodia des Myxomycetes 

 (Memoires de la Societe imperiale des sciences de Cherbourg, vol. XIV). 



^ 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. 



