412 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



nearly or entirely resumed, but when the resistance increases progressively 

 growth is naturally more and more retarded. In the former case, how- 

 ever, more work is done than when growth is entirely unrestricted, just as 

 in the case of a man walking at the same rate in still air and against 

 a strong wind. 



The external work done must reach a maximal value with a definite 

 resistance, since no work at all is done at either extreme, that is, when no 

 growth is possible or when no resistance is applied. The actual amount 

 of external work done affords, however, no criterion as to the internal work 

 involved in growth, and the latter may remain unaltered when the plant 

 is caused to do considerable external work against a resistance 1 . 



Similar considerations apply to tissues -, for in them the slowly grow- 

 ing or inactive cells constitute the resistance which is subjected to tension 

 by the elongation of the more rapidly growing cells. The conditions are, 

 however, somewhat more complicated, since, in part at least, we are dealing 

 with cells capable of growth-responses, and if the stretched tissues continue 

 to grow when the organ is enclosed in a rigid plaster cast, their tension 

 disappears and the compressed tissues act against the cast. This occurs 

 in the nodes of grass-haulms, whereas in many cases the tissue-tensions do 

 not entirely disappear even after prolonged enclosure in a plaster cast. On 

 setting free the organ the original tissue-strains are gradually restored, and 

 the same applies to the individual cells whose walls are again stretched by 

 the internal osmotic pressure. Since, however, the cohesion of the cell-wall 

 is unaltered, no ruptures occur however suddenly the external resistance is 

 removed. 



Curving organs are often able to perform considerable external work. 

 Thus a horizontally-placed shoot may still be capable of a geotropic 

 curvature although it is forced to counteract from four to thirty times the 

 normal mechanical moment, and the curvature is only slightly retarded with 

 a moderate load, although much retarded by a considerable one. Even a 

 slender root may exert a considerable thrust if lateral displacement is 

 prevented, and in the same way shoots are able to break through stiff soil. 



The bursting of the hard coats of seeds, the splitting of the bark by 

 the growth of the wood, and the strangulation of trees by lianes, are all 

 instances in which a considerable external force is exercised as the result 

 of physiological activity. 



The rapid movements of the stamens of Parietaria, the sudden de- 

 hiscence of the fruits of Impatiens and Momordica, are produced by the 

 sudden release of tensions gradually built up during growth. The twisting 

 and untwisting of the fruit-valves of Leguminosae produced by imbibi- 

 tion and drying, as well as the similar movements of the awns of certain 



1 Pfcffer, Brack- und Arbeitsleistungen, 1893, pp. 330, 419. * Id., 1893, pp. 379, 400, 426. 



