TERMINATION, GROWTH, TISSUE TENSION. 157 



of strain thus set up tends to keep the stem rigid and erect. 

 The outer tissue is on the stretch, tending to shorten, while 

 the inner tissue is under compression. 



(a) Cut short longitudinal slits in the cut end of a seedling stem, 

 or the flower-stalk of Dandelion, Tulip, etc. , and set it in water. 

 The slit parts curl outwards, evidently because the inner cells absorb 

 water more rapidly than the outer ones. 



(b) Cut off about 50 cm. from the youngest part of a vigorously 

 growing Elder shoot. Slice off the tissue from two opposite sides, 

 so as to obtain a flat strip the whole length of the original piece of 

 stem. Bisect this strip, and note that each half bends outwards. 



(c) Cut off about 6 cm. of internode from a stout young Elder 

 stem, and measure it accurately. Isolate the pith, by slitting the 

 outer tissue and then removing the hard woody cylinder ; measure 

 the pith, and note that it has become longer on being isolated. 

 Now place the pith in water, and after a few minutes measure it 

 again : it has increased further in length. Next, place the pith in 

 10 per cent, salt solution for some time ; measure and note the 

 decrease in length. Then rinse the pith in water and place it in a 

 large vessel of water for some time : it becomes longer again. 



(d) Cut from vigorously growing shoots (e.g. Elder, Tobacco- 

 plant, Sunflower) some straight young internodes 4 or 5 cm. long. 

 Draw four straight parallel lines on a card, lay an internode on 

 each line in turn, and mark off on the line its two ends. Then 

 remove from the whole length of the internode (1) the epidermis, 

 (2) the cortex, (3) the wood, (4) the pith. Mark off the length of 

 each of the four strips of tissue on one of the four lines, and note 

 that the lengths of the isolated strips of tissue increase from with- 

 out inwards ; as compared with the intact internode, the pith is 

 longer, the epidermis shorter, and the intermediate tissues are of 

 about the same length. 



Hence the pith is in a state of compression, and the epidermis in 

 one of tension. It is sufficient in experiments of this kind to com- 

 pare the lengths of the intact stem, the isolated epidermis, and 

 the isolated pith. The amount of the tension in the intact inter- 

 node may be expressed as a percentage ; if the length of the intact 

 internode is 50 mm., that of the isolated epidermis 49 mm., and 

 that of the isolated pith 54 mm., the tension percentage is 10. 



(e) In the same way determine the lengths of (1) the intact 

 internode, (2) the isolated epidermis, (3) the isolated pith, in 

 several internodes of a growing shoot, and calculate the percentage 

 tension in each iaternode. Note that the tension in the youngest 

 internodes is small, rises in those rather older, and again falls off 

 in the still older internodes. This shows that the longitudinal 

 tension is due chiefly to the turgescence of the pith cells, which 

 absorb much water, so that the pith tends to elongate and therefore 



