148 GERMINATION, GROWTH, TISSUE TENSION. 



mucli richer in dissolved salts. In order to find out how 

 long the stored food lasts, we should therefore use dis- 

 tilled water, so that we know exactly what the roots are 

 supplied with. 



Grow various seeds in jars containing distilled water, 

 fixing them either into holes in muslin or flannel, or into 

 split or bored corks ; fill up the water as required, but 

 always use distilled water. Keep some of them in dark- 

 ness, expose others to the light, and compare their growth 

 and their increase or decrease in dry weight. Another 

 method is to let the roots grow into sand that has been 

 washed thoroughly with tap-water and then with distilled 

 water, using the latter for watering afterwards. 



Seedlings grown with their roots in pure water do not 

 live very long as a rule, especially if they are kept in dark- 

 ness, when their dry weight diminishes, and they die after 

 using -up the stored food. In the light, however, the seed- 

 lings live longer, and for a time increase in dry weight. 

 Bean and Pea seedlings exposed to light, with their roots 

 in distilled water, grow for several months and may even 

 produce flowers, though they are small and weakly as com- 

 pared with seedlings grown in soil. Small seedlings, with 

 scanty food-stores e.g. Mustard may live only a few 

 weeks when exposed to light, with the roots in distilled 

 water, and die still earlier if kept in darkness. 



190. Energy Expended in Growth of Boot and Shoot. 



We know that the radicle and plumule of a Bean seedling, for in- 

 stance, must exert considerable force in growing through the soil 

 the root protected by its cap, the shoot by its recurved tip (or by 

 its pointed form in seedlings like Maize and Wheat). We can 

 roughly measure the force exerted, and by calculation we can 

 roughly determine the amount of energy that is set free by the 

 oxidation of the carbon contained in the seed's store of reserve 

 food. 



The combustion or oxidation of 1 gram of carbon the Broad 

 Bean seed contains roughly 1 gram of carbon sets free enough 

 energy to raise 8 kilograms of water from to 1 C. , and about 

 2 litres of carbon dioxide are given off; if all this energy were 

 used in mechanical work, it would suffice to raise 3,400 kilograms 

 through 1 metre, but the energy is used up by the plant in the 

 form of heat and of chemical work, in addition to mechanical 

 work. 



