138 MASSACHUSETTS HORTICULTURAL SOCIETY. 



chemist's laboratoiy, is accounted for by ttie similarity of their 

 chemical conditions. Thus, starch, gum, and cellulose ma}' recon- 

 vert b}- some change in the arrangement of their constituent atoms, 

 or they may become sugar by the addition of one or two atoms of 

 water. 



Thus we have the ph^'siological action of the leaf, presenting to 

 the scientific mind one of the most interesting pages of nature's 

 hand-book. It is ar. interesting study, not alone because curious, 

 but because it has had no little part to perforin in rendering this 

 world habitable for man. In a brief glance at its action in this 

 connection we recognize its relationship to our soil, which we 

 know to consist of two parts, the mineral and the organic. The 

 mineral is but the disintegrated and pulverized rock, containing, to 

 be sure, many ingredients that enter into the composition of plants, 

 such as potassa, soda, silica, and the universal lime, all more or 

 less soluble in water. But what would our soil be if this were all? 

 It would be but a collection of rock}' hills and sandy wastes, like 

 the great Sahara. But, mingled with the mineral matter, we find 

 the organic matter, which consists of the remains of former tribes 

 of plants and animals, and the products of their decomposition, 

 carbonic acid and ammonia. But, as the earth supported vegetable 

 life before it did animal, we see the important place given to our lit- 

 tle leaf; year after year, century after century, age after age, has the 

 leaf gone on elaborating the juices of plants, extracting carbon 

 from the air and leaving, by the final decay that comes to all, its 

 organism to the soil, making it such as we have it today, a life- 

 supporting element, affording, in its tillage, the occupation of a large 

 majority of civilized men. 



But it is not the soil alone that has been created by the instrumen- 

 tality of our hastily surveyed leaf, for we find within its depths 

 hidden stores of coal and petroleum, enough to last for centuries 

 to come. From whence came they? What has our perishable leaf 

 to do with the formation of coal? It placed it there. That is, it 

 developed the gigantic pines, ferns, and lycopodiums from which it 

 was composed, and in doing so may have had much to do with 

 rendering our atmosphere respirable for animals. We are indebted 

 to M. Brongniart for a very ingenious hypothesis which he has 

 raised upon tlie fact that'an increased quantity of carbon may, un- 

 der peculiar circumstances, be assimilated by the leaf He supposes 



» Carpenter's Principles of Comparative Physiology. 



