156 GERMINATION, GROWTH, TISSUE TENSION. 



(2) Take six germinating Beans with roots from 2 to 

 3 cm. long ; mark each root with a transverse ink line 

 at 1 cm. from the tip. Fix the seeds by long pins to the 

 corks of two tall wide-mouthed jars, placing three seeds in 

 each jar. Fill A with water, so that the seeds are sub- 

 merged ; in B place only a little water, so that the seed- 

 lings will be growing in damp air. Measure the roots 

 again after a day or two. Then fill up B with water, and 

 note that the rate of growth of the roots is diminished 

 during the succeeding days. 



IY. TURGOR, TISSUE TENSIONS. 



206. Wilting due to Flasmolysis. (a) Pull up 

 whole seedlings or cut off their shoots, and let them lie on 

 the table ; they become limp (wilted, flaccid), and it is easy 

 to prove (e.g. by weighing before and after) that they have 

 lost water in wilting. Put the limp shoot into water ; it 

 becomes firm again. 



(6) Cut off the shoot of a seedling and put it into 5 per 

 cent, salt solution. When the shoot has become limp, wash 

 it under a tap, set it in water, and note that it turns firm, 

 (turgescent) again. 



(c) The shoots used in these two experiments are not 

 necessarily killed unless they have been allowed to become 

 dry, or unless the salt solution is too strong or they have 

 been kept in it too long. Prove this by pulling up whole 

 seedlings, making them flaccid by means of salt solution, 

 and re-planting them in wet sawdust or soil. 



207. Longitudinal Tissue Tension. In addition to 

 the three supporting or " skeletal " tissues wood- vessels, 

 sclerenchyma, collenchyma the ordinary thin-walled tissue 

 (parenchyma) plays an important part in maintaining the 

 rigidity of herbaceous stems, as well as of petioles, leaf- 

 blades, and flower-stalks, by the turgidity of its cells. In 

 a herbaceous stem the pith has a strong tendency to elon- 

 gate, but this is hindered by the outer tissue, and the state 



