158 GERMINATION, GROWTH, TISSUE TENSION. 



to stretch the extensible outer tissues, but the latter are elastic 

 and therefore tend to compress the pith. As the stem grows older, 

 the pith loses its water and stops growing. Hence the longitudinal 

 tension disappears, but in its place there appears transverse tension 

 ( 208). 



(/) By finding what strength of salt or sugar solution is needed 

 to bring about plasmolysis, we get a rough idea of the osmotic force 

 of the cell-sap. Saltpetre solutions are generally used; a 1 per cent, 

 solution of this salt (nitrate of potash, KN0 3 ) exerts a pressure of 

 3| atmospheres. 



(g) Split a Dandelion stalk longitudinally into four strips and 

 notice that each strip at once becomes curved, with the epidermis 

 on the concave side : why ? Place some strips in water, others in 

 strong (about 10 per cent.) salt solution, and observe the differences 

 in the curvature caused by the changes in the turgidity of the inner 

 tissue i.e. that nearest the centre of the stalk. 



(h) Cut a long narrow strip of Dandelion stalk and fasten the 

 ends securely, by threads or pins, close together to a piece of wood. 

 Dip the strip into water and carefully watch how it coils ; part of 

 it twists in one direction, part in the opposite direction, and be- 

 tween these there is a part where the spiral reverses. This gives 

 an excellent illustration of the coiling of a tendril, which shows a 

 similar reversed spiral when the free end has become fixed to a 

 support. 



(i) Split a Dandelion stalk and cut the curled-up strips into 

 rings. If the ring is placed in water it will become more tightly 

 coiled ; if in a very strong solution of salt or sugar, it will open 

 out. In this way we can find out what strength of solution pro- 

 duces neither increase nor decrease of curvature and therefore 

 equals the osmotic force of the soft tissue, i.e. the osmotic strength 

 of the cell- sap. 



(j) Prepare a 5 % solution of common salt, by stirring 25 grams 

 of salt into 500 c.c. of water. Get ten saucers ready, and into one 

 pour 100 c.c. of the solution. Then, using a graduated beaker, 

 dilute the 5 % solution with water, so as to make 4 %, 3%, 1 %, 0'5 #, 

 0'4#, 0*3 %, 0'2#, and 0*1 % solutions, pouring 100 c.c. of each into 

 one of the saucers. In each saucer place two or three rings, and 

 find out in which saucer the rings become neither more nor less 

 curved. For comparison place some rings into a saucer containing 

 plain water. 



(k) Measure an "internode" of young Sunflower stem, then 

 extract the pith by using a cork-borer, and measure (1) the isolated 

 pith, (2) the outer tissue : the former has elongated, the latter 

 contracted. 



(1) Another and simpler method is to use the long leaf-stalks of 

 Rhubarb or of "Arum Lily." Lay the stalk down, cut the ends 



