DISPERSION AND GERMINATION OF SEEDS. 225 



of nature, that there can be no doubt that the seed from a 

 single thistle or dandelion would, in the course of a few years, 

 be sufficient to cover with plants not only every square inch of 

 the superficies of our own world, but the entire surface of every 

 other planet in the solar system. 



But although nature has been thus careful to ensure a repe- 

 tition of their beautiful and evanescent forms, all plants 

 multiply within prescribed limits, which they cannot pass. 

 Fecundity is therefore no barrier to the variety which every- 

 where prevails, which is the principal charm of the vegetable 

 creation, and from which we derive so much instruction in the 

 study of their individual forms. 



When, however, the seed falls into a soil favorable to its 

 germination, it will grow and become a plant, running through 

 all the phases of the vegetation of its predecessor. 



We have only now to lay before the student the conditions 

 which are necessary to germination, and the interesting series 

 of phenomena connected with the evolution of the young 

 plantule from its integuments. 



The exterior agents indispensable to germination are water, 

 air, and heat. 



Water is necessary in germination as in all the other pheno- 

 mena of vegetable life. It penetrates into the substance of the 

 seed, softens its envelopes, and makes the embryo swell. It 

 therefore places the seed in the conditions which are most 

 favorable for its development. As soon as germination com- 

 mences, it dissolves the dextrine and the other soluble prin- 

 ciples which exist in the seed, and which are formed by the 

 transformation of the starch, and conveys these nutritive mate- 

 rials to the young embryo. 



Air as well as water is also necessary. Experiments show 

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