DISPERSION AND GERMINATION OP SEEDS. 227 



to dextrine, and the continuation of the same process con- 

 verting some dextrine into sugar, which being dissolved by 

 the water, also penetrates all the parts of the embryo, thus 

 induces the necessary nutritive and germinating processes. 



The starch contained within the folds of the seed, is there- 

 fore at the end of a certain time, completely re-absorbed. 

 This disappearance of the starch is the result of its combus- 

 tion by the embryo, or of its slow conversion principally into 

 carbonic and other vegetable acids, and in part into cellulose. 



Heat is therefore evolved during germination, and a certain 

 amount of it becomes indispensable to the vital action of the 

 seed. Placed in the midst of a temperature below zero, it 

 remains benumbed and stationary even under the influence of 

 air and humidity. But a mild temperature accelerates the 

 development of all the phenomena of vegetation. Hence it is 

 that the gardener is accustomed to hasten the development of 

 such exotic grains as it is his interest to cultivate, by sowing 

 them in a hot bed, and by consequence surrounding them with a 

 humid and artificial heat. But it is necessary that this tem- 

 perature does not pass certain limits, otherwise, so far from 

 hastening the development of the seeds, it will dry up and 

 destroy the principle of life within them. 



The degree of heat required to excite the vitality of the 

 embryo, varies from 50 to 80 (Fahrenheit,) for the plants of 

 temperate climates. The seeds of tropical plants require a 

 much higher heat to call them into action, varying from 90 to 

 110 (Fahrenheit,) and occasionally a more elevated tempera- 

 ture than even this, is found to be necessary. 



Light exercises an unfavorable influence on germination. 

 This must necessarily be its effect on the germinating seed, for 

 we have shown that the absorption of the carbonic acid of the 



