THE STIMULUS OF TENSION 127 



SECTION 36. The Stimulus of Tension. 



Increased tension often causes an organ to increase greatly in strength. 

 Thus Hegler 1 found that the hypocotyl of a seedling of Helianthus 

 annmiS) having an original breaking strain of 160 grammes, was able to 

 bear 250 grammes after it had been stretched by a weight of 150 grammes 

 for two days. Under the influence of further loads of 350 and 300 grammes 

 the tensile strength rose in a few more days to 400 grammes. The petiole 

 of Helleborus niger ^ with a breaking strain of 400 grammes, was able to bear 

 a breaking strain of 3-5 kilogrammes after it had been subjected to increasing 

 loads for five days, whereas an unloaded petiole underwent little change in 

 the same time. In the case of the hooks of certain tropical climbers, loads 

 of from oi to i gramme may produce a distinct effect upon the growth 

 and strength of the attaching organ 2 , whilst as the result of tension the 

 flattened base of the tendril of Bauhinia becomes nearly circular in 

 outline (Ewart, 1. c., p. 222). 



This power of response seems to be possessed by most young organs 

 capable of growth, for positive results have been obtained with the stems, 

 petioles, and peduncles of a variety of plants, with tendrils and also with 

 the internodes of Chara 3 . That the strength of an attaching or supporting 

 organ should increase within certain limits according to the strains to 

 which it is subjected is of great biological importance. The influence 

 of use and disuse upon the development of the muscles, and even of the 

 bones of animals, is well known 4 . 



In plants the increase of strength is commonly due merely to an 

 increase in the thickness or in the elasticity of the walls of pre-existent cells, 

 and naturally the reaction is most prominently shown in the mechanical 

 tissues. Hegler often observed a more abundant development of col- 

 lenchyma, or an increase in the thickness of the walls of these or of 

 sclerenchyma cells, as the result of increased tension. In the petiole of 

 Helleborus niger strong tension causes large numbers of thick- walled bast- 

 fibres to be formed from phloem elements which usually remain thin-walled 5 . 



Neapel, 1882, Bd. in, p. 431) states that the water-pressure exercises no noticeable influence upon 

 the distribution of marine algae. [It may be doubted whether this statement applies to such floating 

 forms as Sargassum, or to the fixed forms provided with air-bladders (Fucus, Ascophylluni] which are 

 specially adapted for life between the tide-marks. The pressure cannot be entirely without effect, 

 since it influences the physical surroundings of the plant, but the illumination is undoubtedly a far 

 more potent factor. In any case Melsens' results need confirmation. A pressure of 8,000 atmospheres 

 corresponds to that of 50 miles of sea-water, or 50 tons per sq. inch.] 



1 See Pfeffer, Sitzsb. d. Sachs. Ges. d. Wiss., 1891, p. 639. Similar effects have been observed by 

 Vochting (Nachr. d. k. Ges. d. Wiss. zu Gottingen, 1902, Heft 5, pp. 5, 6), though not always to the 

 expected extent, and in decapitated stems hardly at all. 



2 Ewart, Ann. du Jard. Bot. de Buitenzorg, 1898, T. XV, pp. 193, 200, 203, 207. 



3 Richter, Flora, 1894, p. 418. 



* Hertwig, Die Zelle u. d. Gewebe, 1898, p. 106. 



5 Thick-walled bast-fibres sometimes appear in the petiole of Helleboms niger when growing 



