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140 ‘PROPAGATION OF PLANTS. 
For this aiviits, thoy must be carefully excluded from the 
action of heat and moisture, and other chemical agents. 
552. Four conditions are necessary for the process of ger 
mination,—the presence of water, of heat, and of air, and the 
exclusion of light. 
553. Water most probably acts in four ways; it softens — 
the integuments and renders them capable of being burst by 
the swollen embryo : dissolves the nutritive matter contained 
in the seed, thus reducing it to a fit state for being absorbed for 
the nutrition of the embryo; conveys in solution nutritive 
particles from other sources : and by being decomposed, fur: 
nishes two important ingredients in the composition of ve- 
getables. 
554. The air, by means of the oxygen which it contains, 
effects a chemical change on the farina of the albumen, °F 
that deposited in the cotyledons. The oxygen combines with — 
the carbon, and forms carbonic acid, which escapes} thus, 
the proportion of oxygen and hydrogen being increased by 
the expulsion of the carbon, the farina is converted into § 
semi-fluid substance, of a saccharine or mucilaginous nature, 
consisting of starch, gum and sugar, well adapted for the 
nutrition of the plant in its infant state. - 
555. Heat always promotes chemical combination and de- 
composition, and thus assists the action of the water in dis- ‘4 
solving the hard parts of the seed, and that of the air in 18 
part of the process. Most probably heat also acts a he 
general stimulus to the absorbents in the seed. Seeds can 
not be made to germinate in very cold weather, except P 
the application of artificial heat. a 
556. Light is unfavourable to germination, because it Us 
___ poses to an accumulation of carbon in the seed, and a con 
sequent hardening of the parts; or rather prevents the ex 
 pulsion of carbon, and consequent softening of the parts, 
_ which is necessary, that they may be taken up and applied to 
the use of the plant. 
_ 557. From the operation of these causes, it will be 
