210 ELEMENTS OF SCIENCE 



bean." Meanwhile the pistil, which encloses the growing 

 ovules, itself rapidly enlarges into a pod, which, when 

 ripe, bursts and sets free the ovules which have now 

 become seeds, after which it decays. 



When the seed, the bean, finds its way to the earth, 

 under fixed conditions of warmth and moisture, it 

 germinates. This process of germination consists in the 

 bursting of the seed-coat by the swelling cotyledons, 

 which become green and emerge as fleshy leaves, while 

 the miniature stem ascends and the little root descends, 

 both meantime absorbing nutriment from the cotyle- 

 dons. 



The stem of the adult bean plant is of complex 

 structure, its component cells having become modified 

 into different tissues. 



As to the physiology of the bean plant, it may be 

 divided into (i) the functions which minister to the 

 preservation of the individual and (2) those which 

 concern the preservation of the race. 



Liquid is absorbed by the roots and part of it evapo- 

 rates, by what is called transpiration, from the surfaces 

 of the leaves, whence air and vapour are conveyed 

 inwards through their stomata.* Thus a sort of 

 circulation also takes place. Liquid ascends between 

 the fibres of the stein, being drawn up through 

 evaporation from the leaves and being pushed up 

 by the absorption of the roots. Water with suitable 

 salts t in solution (nitrates of potassium or of calcium and 

 sulphates of iron or of magnesium) is absorbed by the 

 roots, while carbon is fixed by the decomposition of 

 carbonic acid, during daylight, by the green leaves. 

 These processes result in the formation of cellulose, 



* See ante, p. 204. t See ante, p. 134. 



