Various Physical Factors 351 



supporting its own weight. Plants thus treated were definitely weaker 

 than the controls. No structural changes are reported. The upward pull, 

 weak as it was, may have stimulated growth in length and thus tended 

 to make the plant top-heavy. 



Newcombe (1895) supported the base of the stem in young sunflower 

 and squash plants by encasing it in plaster, thus relieving it of mechani- 

 cal strain. In that part of the stem under the cast much less mechanical 

 tissue was formed, although this developed rapidly when the cast was 

 removed. Other effects of such treatment, especially on respiration, might 

 account for the results obtained. 



The studies on experimental traction and compression are contra- 

 dictory and indecisive, and many factors other than mechanical ones 

 may well be involved. Little work has been done in this field in recent 

 years. Schwarz ( 1930 ) subjected the problem of mechanical factors in 

 development to critical review and concluded that they have little effect 

 and that the results attributed to them may well be due to nutritional 

 influences and transpiration. Rasdorsky (1931), however, took issue 

 with him strongly. 



Bending and Swaying. There is much more agreement as to the effect of 

 bending and swaying plant organs. Most workers find that, when grow- 

 ing herbaceous stems are bent, the cells (especially collenchyma and 

 bast fibers) on the convex side are smaller in cross section and thicker- 

 walled than corresponding ones on the concave side. The same results 

 are evident in plants grown on a clinostat, showing that gravity is not 

 involved. 



A good discussion of this problem is presented by Biicher ( 1906 ) . To 

 this result of bending he gave the name camptotrophism. Since cells on 

 the convex side of the bend are evidently under tension and those on the 

 concave side under compression, the histological differences observed 

 seem related to the type of mechanical strain involved. These effects 

 agree with the ones from the experiments just reported where tension 

 seems to increase wall thickness and reduce cell size, and compression 

 produces the opposite result. Biicher obtained more direct evidence in 

 support of this conclusion. He enclosed in plaster most of the lower 

 portion of a growing hypocotyl of Ricinus. The upper part was enclosed 

 in another casing of plaster, leaving a short portion of hypocotyl unen- 

 closed between the two. The weight of the upper layer of plaster was 

 supported by the hypocotyl, which was thus subjected, particularly in 

 its free portion, to considerable lengthwise compression (Fig. 16-5). In 

 the control plants the hypocotyl had relatively small and thick-walled 

 cells. In the compressed hypocotyls, however, the cells were much 

 larger and had very thin walls. This sort of experiment seems worth re- 

 peating with modern techniques of analysis. What, for example, would 



