176 THE GROWTH OF ROOTS AND STEMS. 



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 % solution of this salt 

 (nitrate of potash, KN0 3 ) exerts a pressure of 3^ atmospheres. 



The osmotic pressure within living cells is often much greater than 

 this. In the pith of Sunflower it is about 13 atmospheres ; that is, a 

 pressure of 13 atmospheres would be needed to prevent the isolated 

 pith from expanding. In the pulvinus, or motor organ, of a Scarlet 

 Runner leaf the pressure is about 11 atmospheres; in the cambium of 

 trees about 15 atmospheres, and in their medullary rays about 20 ; in 

 Onion bulbs and in Beet-root (which contain large amounts of sugar) 

 about 20 atmospheres ; and in the " nodes " of Grasses about 40 atmo- 

 spheres, a pressure as great as that of the steam in a powerful engine. 



* (6) 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. 



(c) 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 between 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. 



* (d) 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 produces 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. 



* (e) Prepare a 5 % solution of common salt, by stirring 25 grammes of 

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

 100 c.cs. 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.cs. 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. 



(/) What would happen if transverse rings were used ? With a dry 

 razor cut sections across a Dandelion stalk and slit each ring at one 

 point. Put some of the rings into water, others into salt solutions of 

 different strengths. In water the rings become more curved in this 

 case, because the pith-cells sink in the tangential direction. 



(g) 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. 



