226 COMPOUND ORGANS OF PLANTS. 



that seeds will not germinate in vacuo, or in a space from 

 which the air has been artificially removed. Hence seeds 

 buried too deeply in the soil will not germinate, as the air 

 cannot get access to them, but if by any natural or artificial 

 causes they arc brought into the superficial beds, germination 

 very soon commences. It is thus that we see suddenly appear 

 in a locality certain plants which did not grow there before, as 

 for instance, when a waste field is cultivated. 



It is the oxygen of the air which acts principally in germina- 

 tion ; for seeds plunged in pure nitrogen or hydrogen neither 

 germiaatc nor develope. It is by the absorption of oxygen 

 that the starch which remains in the nucleus, or which has been 

 absorbed into the cotyledons, is rendered soluble and nutritive. 

 It is well known that starch is quite insoluble in cold water. 

 By what remarkable operation; in vegetation does it become 

 soluble, so as to be dissolved and transported to all parts of the 

 plant? This question we now proceed to answer. Starch is 

 profusely spread through all the organs of the plant, and is 

 accumulated especially in the seed, as a store of nutriment on 

 which the young embryo subsists, until such times as its roots 

 and leaves are sufficiently developed for the accumulation of its 

 proper food from the earth and atmosphere. But in order 

 to its assimilation by the young embryo it is necessary that 

 the starch in the cotyledons or nucleus should be rendered 

 soluble. When the temperature and other conditions are favor- 

 able, a vegetable secretion termed diastase forms itself in all the 

 cells which contain starch. This diastase possesses the sin- 

 gular property of transforming starch into a soluble gum 

 termed dextrine, which the water is able to carry to all parts 

 of the plant. The action of the oxygen of the air, through 

 the secretion of the diastase, having thus changed the starch 



