160 GERMINATION, GROWTH, TISSUE TENSION. 



211. Extensibility and Elasticity of Tissues. Cut 



fresh pieces of stein of Elder, Honeysuckle, Vine, or 

 Aristolochia. Make a transverse mark with. Indian ink at 

 the upper and the lower ends of (A) a young internode 

 near the apex of the shoot, (B) an older internode. Now 

 lay the stem against a scale, and stretch it as much as 

 possible without breaking it. Note that the younger inter- 

 nodes are much more extensible than the older ; that the 

 stem shortens again when left to itself after having been 

 stretched hence the tissues are more or less elastic ; and 

 that they do not regain their original length, but remain 

 permanently longer hence the tissues are incompletely 

 elastic. 



212. Flexibility of Tissues. That growing tissues 

 are flexible, but incompletely elastic, is easily shown. 

 From one of the plants just named choose a straight 

 flexible internode. Mark a card with concentric circles, 

 and bend the stem over the card until its axis coincides 

 with one of the circles : note the radius of curvature. 

 Leave the stem to itself for some time ; it does not become 

 straight, but remains permanently bent determine its 

 radius of curvature. 



213. Relation between Turgidity, Growth, and 

 Extensibility. Determine the distribution of growth in 

 the shoot and root of a seedling ( 199, 200). Then lay 

 the seedling in a 10 per cent, solution of salt ; after an 

 hour or two the tissues will be completely plasmolysed. 

 Measure the zones again, and note that they have become 

 shorter through loss of turgescence, and that the plas- 

 molysis is greatest in the zones which have been growing 

 most rapidly. 



The preceding experiment suggests that the rate of growth of the 

 cells depends on the amount of their turgor tension. This tension 

 is determined by (1) the amount of the osmotic pressure and (2) the 

 amount of the resistance offered by the stretched cell- walls owing to 

 their extensibility. 



Take a Phaseolus seedling with epicotyl about 4 cm. long ; mark 

 it into zones 5 mm. long ; after two days, measure the zones again, 



