64 EXPERIMENTS WITH PLANTS 



Carefully weigh and measure a dry seed, place it 

 in water for twenty -four hours, and then measure it 

 again. Now place it in a very strong solution of salt 

 or sugar until it returns to its original weight, and 

 remeasure. Does it return to its original volume ? 

 After the seed has grown for a time it is not possi- 

 ble to shrink it back (by means of strong solutions 

 or by drying) to its original volume, even if we suc- 

 ceed in bringing it back to its original weight. The 

 growth has become fixed, or "set." The seed is com- 

 posed of very small chambers or cells (as may be 

 easily seen by examining the broken surface of a 

 Horse-bean seed-leaf with a hand-lens), each of which, 

 like our apparatus, contains substances which absorb 

 water and so generate pressure by which the walls of 

 the chamber are distended (just as was the rubber 

 fastened to the end of the tube in our apparatus). 

 The rubber returns to its original size as soon as the 

 pressure ceases. The stretched walls of the cells 

 behave in the same way at first, but after a time they 

 are strengthened and reinforced by the deposit of 

 more material ; when this has occurred the pressure 

 may be removed and, while they will collapse to a cer- 

 tain extent, they will not shrink back to their original 

 size. The growth may then be said to be "set." 

 We may ascertain when this occurs by the method 

 described above. 



The explanation which has been given of the 



