386 IDIOPHYSICS. 



force, or by the agency of animals, or the wind ; and thrown into 

 situations more or less favorable for germination; that is, for the 

 development of a new plant of the same kind. The necessary con- 

 ditions for germinating or sprouting, are warmth, moisture, air, and 

 the exclusion of light: though light is afterwards beneficial to their 

 growth. Porous earth admits the air, but excludes the light, if the 

 seeds are not planted too deep. The corcle, or embryo, then ex- 

 pands into a radicle, which shoots downwards to form the root ; and 

 a plumula, which shooting upwards, becomes the stem of the future 

 plant: while the cotyledons are generally converted, by fermentation, 

 into sugar, for its nourishment. Plants may be propagated, not only 

 by reproduction from the seed ; but also by continuation ; whether 

 by grafting, inoculating, transplanting, or setting out shoots or twigs 

 in the earth. 



Plants are nourished, partly by the roots, which absorb moisture, 

 and nutritious salts, especially those of potassa ; and partly by the 

 leaves, which absorb carbonic acid, and perhaps oxygen, from the 

 air. Their carbon is doubtless derived from the decomposition of 

 carbonic acid ; and their hydrogen from water : while a part of the 

 oxygen is set free. The leaves, are to plants, what the lungs are to 

 animals ; but they exhale oxygen, instead of carbonic acid, at least 

 when exposed to the light of the sun. Whether this action is 

 reversed by night, and carbonic acid evolved, we are not prepared to 

 say ; as authorities differ thereon. In regard to the mode of growth, 

 plants are divided into two great classes. In monocotyledonous or 

 endogenous plants, the stem has seldom any external bark, but 

 consists mostly of pith ; and the growth takes place at the centre ; 

 while the preexisting parts are thereby crowded outwards, and 

 compressed, as in the palm tree, and Indian corn. But in dicotyle- 

 donous, or exogenous plants, the growth takes place externally, 

 between the wood and the bark, as in the elder, or oak. The stem 

 of exogenous plants, consists of the pith, or soft central core ; the 

 heart wood, or hardest part, and the sap wood, or alburnum, which 

 is the outer and softer part of the wood ; the liber or inner bark, 

 strong and fibrous ; the cellular integument, or middle bark ; and 

 the cuticle, or outer bark, probably lifeless, and serving merely to 

 protect the parts within. 



The substance of plants, is generally porous, and consists of either 

 cellular or vascular texture; the former composed of cells, and 

 existing in the pith, bark, and leaves ; while the woody part is 

 chiefly vascular, consisting of tubular fibres. The sap of plants, is 

 a limpid. liquid, which rises from the roots to the leaves, it is said, 

 through the medullary sheath, consisting of spiral vessels surround- 

 ing the pith ; and after respiration, it descends through the liber, or 

 inner bark, underneath which it deposits the camb, a mucilaginous 

 substance ; and from this a new annual layer of wood is formed, by 

 which the age of exogenous plants may be known. The proper 

 juices, or peculiar fluids, secreted by plants, as gum, oils, and the 

 like, are said to be elaborated by glandular knots, analogous to the 

 glands of animals. 



3. Under the head of Systematic Botany, we proceed to 



