94 PLANT PHYSIOLOGY 



tissue is, in part at least, to be attributed to the action of gravity, although 

 tension and pressure may also take part to some extent. This leads us to 

 consider more in detail the influence of mechanical factors on growth. 



1. 43, before Tension of necessity, &c. read Owing to mechanical 

 stoppage of growth the meristem cells are able to remain for long unaltered, 

 and to start growing again after the cause of stoppage has been removed. Cells 

 which are in the act of elongating gradually turn into permanent tissue, and 

 hence often remain smaller and less differentiated than under normal condi- 

 tions (NEWCOMBE, 1894). 



1. 49 P. 315, 1. 13, for The new cell-walls . . . surface, read In the second 

 place we have to inquire whether tension and pressure have any effect on the 

 formation of the tissues. Since it is possible to observe in ripening fruits an 

 increasing differentiation of mechanically active elements (comp. KELLER, 

 1904), it might be supposed that this was due to the influence of tension or 

 pressure ; but many experiments (VocHTiNG, 1902 ; WIEDERSHEIM, 1902 ; 

 BALL, 1903 ; KELLER, 1904) have shown that a uniform longitudinal tension 

 or pressure has no such effect (comp. WILDT, 1906). It is all the more surprising 

 that in violent curvatures and in geotropic curvings which are prevented from 

 taking place (BALL, 1903 ; BUCHER, 1906) peculiar anatomical changes do 

 appear. In violent bending of shoots which have the power of growth, the 

 walls of the collenchyma, sclerotic, and xylem elements on the convex side 

 increase in thickness, and their lumina decrease ; on the other hand, in stems 

 whose geotropic curvature has been mechanically prevented, we meet with the 

 same sort of formation of tissues on the upper side, i. e. on the side which would 

 have become concave had it not been prevented from bending. Generally 

 speaking, in both experiments the places where cell-wall formation is most 

 vigorous are under tension, while on the opposite sides, under pressure, the 

 tissue elements are thin-walled and have wide lumina. Although the plant 

 does not react by special tissue formation to a mechanical influence in the 

 longitudinal direction, when that influence is uniform, it does so if differences 

 in tension are induced. Further, lateral organs frequently exhibit a contrast 

 in structure on the concave and convex sides. In curved roots especially 

 (NOLL, 1900) one sees the lateral rootlets appearing exclusively on the convex 

 sides. NOLL believes that the difference in tension between the two sides is 

 not the cause of the unilateral formation of secondary roots, and he considers 

 that the plant must react to the curvature as such in the way mentioned. If, 

 however, the diarch root of a lupin be divided longitudinally in such a way 

 that each half contains one vascular strand, and hence one portion of the rhizo- 

 genic layer, one finds that the lateral roots develop in quite the same way 

 whether the parent root be concave, convex, or straight. From this fact the 

 conclusion must be, in our opinion, drawn that it is the difference in tension 

 between the rhizogenic layers that leads to unilateral formation of lateral roots. 



From what has been said already we know that cell division is also affected 

 by mechanical influences ; new cell-walls, unless prevented by other agencies, 

 arrange themselves parallel with the line of pressure and at right angles to that 

 of tension (KNY, 1901). Further, one very remarkable instance of stimulatory 

 activity is met with when closely associated parts of the plant are affected by 

 pressure acting with different intensity. A case of this kind is ' contact pres- 

 sure ' due to hard bodies, more especially such as have rough surfaces. Sensi- 

 tivity to contact is a phenomenon which we shall meet with in certain cases of 

 movement, but not a few examples of it are forthcoming in the formation of 

 organs. Thus, for example, root-hairs have their growth inhibited by contact 

 with soil particles, and they adapt themselves in the most intimate manner to 

 the inequalities of the particles with which they are in contact. 



