30 OF LIFE, AND ITS CONDITIONS. 



in the fact that this evolution is carried on at the expense of organic com- 

 pounds already prepared by extrinsic agency, until (the store of these 

 being exhausted) the young plant is sufficiently far advanced in its develop- 

 ment to be able to elaborate them for itself, the condition of the germinat- 

 ing embryo resembles that of an Animal. Now the seed, as already pointed 

 out ( 3 note), may remain (under favorable circumstances), in a state of 

 absolute inaction during an unlimited period. If secluded from the free 

 access of air and moisture, and kept at a low temperature, it is removed 

 from all influences that would on the one hand occasion its disintegra- 

 tion, or on the other would call it into active life. But when again exposed 

 to air and moisture, and subjected to a higher temperature, it either germi- 

 nates or decays, according as the embryo it contains has or has not preserved 

 its vital endowments, a question which only experiment can resolve. The 

 process of germination is by no means a simple one. The nutriment stored 

 up in the seed is in great part in the condition of insoluble starch ; and this 

 must be brought into a soluble form before it can be appropriated by the 

 embryo. The metamorphosis is effected by the agency of a ferment termed 

 diastase; which is laid up in the immediate neighborhood of the embryo, 

 and which, when brought to act on starch, converts it in the first instance 

 into soluble dextrin, and then (if its action be continued) into sugar. The 

 dextrin and sugar, combined with the albuminous and oily compounds also 

 stored up in the seed, form the "protoplasm" which is the substance imme- 

 diately supplied to the young plant as the material of its tissues; and the 

 conversion of this protoplasm into various forms of organized tissue, which 

 become more and more differentiated as development advances, is obviously 

 referable to the vital activity of the germ. Now it can be very easily shown 

 experimentally that the rate of c/roivth in the germinating embryo is so closely 

 related (within certain limits) to the amount of Heat supplied, as to place 

 its dependence on that agency beyond reasonable question ; so that we seem 

 fully entitled to say that Heat, acting through the germ, becomes the con- 

 structive force or power by which the Vegetable fabric is built up. 1 But 

 there appears to be another source of that power in the Seed itself. In the 

 conversion of the insoluble starch of the Seed into sugar, and probably also in 

 a further metamorphosis of a part of that sugar, a large quantity of carbon 

 is eliminated, by combining with the oxygen of the air so as to form car- 

 bonic acid; this combination is necessarily attended with a disengagement 

 of heat, which becomes very sensible when (as in malting) a large number 

 of germinating seeds are aggregated together; and it cannot but be regarded 

 as probable that the heat thus evolved within the seed concurs with that de- 

 rived from without, in supplying to the germ the force that promotes its 

 evolution. 



9. The condition of the Plant which has attained a more advanced stage 

 of its development, differs from that of the germinating embryo essentially 

 in this particular, that the organic compounds which it requires as the ma- 

 terials of the extension of the fabric are formed by itself instead of being 

 supplied to it from without. The tissues of the green surfaces of the leaves 

 and stems, when acted on by light, have the peculiar power of generating at 



1 The effect of Heat is doubtless manifested very differently by different seeds; such 

 variations being partly specific, partly indhnifmif. But these are no greater than we 

 see in the Inorganic world; the increment of temperature and the augmentation of 

 bulk exhibited by different substances when subjected to the same absolute measure 

 of heat, being as diverse as the substances themselves. The whole process of "rmi It- 

 ing." it may be remarked is based on the average regularity with which the seeds 

 of a particular species may be nt any lime forced to a definite rate of germination by 

 a definite increment of temperature. 



