2'28 



SCIENCE OK AC l!i CULTURE. 



Part II. 



from tlic prolongation of the radicle [Jig. 188. a), 

 bursting through its proper integuments, and direct- 

 ing it-- extremity downwards into the soil. The next 

 sU'|> in the process of germination i-- the evolution of 

 the cotyledon or cotyledons (c), unless the seed isal- 

 together acotyledonous, or the cotyledons hypogean, 

 as iii the oak (/)). The next step, in the case of seeds 

 furnished with cotyledons, is that of the extrication 

 of the plumelet (c), or first real leaf, from within the 

 cotyledon or from between the cotyledons, and its 

 expansion in the open air. The developement of the 

 rudiments of a stem(d), if the species is furnished 

 with one, is the last and concluding step, and the 

 plant is complete. Whatever way the seed may be 

 deposited, the invincible tendency of the radicle is to 



descend and fix itself in the earth ; and of the plumelet, to ascend into the air. Many 

 conjectures have been offered to account for this. Knight accounts for it on the old 

 hut revived principle of gravitation. Keith conjectures that it takes place from a power 

 inherent in the vegetable subject, analogous to what we call instinct in the animal sub- 

 ject, infallibly directing it to the situation best suited to the acquisition of nutriment and 

 consequent developement of its parts. 



1580. The chemical phenomena of germination consist chiefly in the changes which are effected in the 

 nutriment destined for the support and developement of the embryo till it is converted into a plant 

 This nutriment either passes through the cotyledons, or is contained in them; because the embryo il. B 

 when they are prematurely cut off! But the farinaceous substance of tin- cotyledons, at least in exal- 

 buminous seeds, is a proof that they themselves contain the nutriment. They are to be regarded, therefore, 

 ,„ repositories of the food destined for the support of the embryo in its germinating state ; and, if the 

 seed is furnished with a distinct and separate albumen, then is the albumen to be regarded as the repo- 

 sitory of food, ami the cotyledon or cotyledons as its channel of conveyance. But the food thus contained 

 in the albumen or cotyledons is not yet fitted for the immediate nourishment of the embryo: some 

 previous preparation is necessary; some change must be effected in its properties. This change is 

 effected by the intervention of chemical agency. The moisture imbibed by a seed placed in the earth is 

 immediately absorbed by the cotyledons or albumen, which it readily penetrates, and on which it imme- 

 diately begins to operate a chemical change, dissolving part of their farina, or mixing with their oily 

 particles, and forming a sort of emulsive juice. The consequence of this change is a slight degree of 

 fermentation, induced, perhaps, by the mixture of the starch and gluten of the cofyledor.s in the water 

 which they have absorbed, and indicated by the extraction of a quantity of carbonic acid gas, as weli as 

 by the smell and taste of the seed. This is the commencement of the process of germination, which 

 takes place even though no oxygen gas is present. But if no oxygen gas is present, then the process 

 stops; which shows that the agency of oxygen gas is indispensable to germination. Accordingly, when 

 oxygen gas is present, it is gradually inhaled by the seed; and the farina of the cotyledons is found to 

 hive changed its savour. Sometimes it becomes acid, but generally sweet, resembling the taste of sugar ; 

 and is consequently converted into sugar or some substance analogous to it. This is a further proof that 

 a degree of Fermentation has been induced ; because the result is precisely the same in the process of the 

 fermentation of barley when converted into malt, as known by the name of the saccharine fermentation ; 

 in which oxygen gas is absorbed, heat and carbonic acid evolved, and a tendency to germination indi- 

 cated b\ the shooting of the radicle. The effect of oxygen, therefore, in the process, is that of converting 

 the farina of the albumen or cotyledons into a mild and saccharine food, fit for the nourishment of the 

 infant plant by diminishing the proportion of its carbon, and in augmenting, by consequence, that of its 

 oxygen and hydrogen. The radicle gives the first indications of life, expanding and bursting its integu- 

 ments, and at length fixing itself in the soil: the plumelet next unfolds its parts, developing the rudi- 

 ments of leaf, branch, and trunk: and, finally, the seminal leaves decay and drop off"; and the embryo 

 has been converted into a plant, capable of abstracting Immediately from the soil or atmosphere the 

 nourishment necessary to its future growth. 



Sect. II. Food of the vegetating Plant. 



1521. The substances which plants abstract from the soil or atmosphere, or the food of the 

 vegetating plant, have long occupied the phylological enquirer. What then are the com- 

 ponent principles of the soil and atmosphere? The investigations and discoveries of 

 modern chemists have done much to elucidate this dark and intricate subject. Soil, in 

 general, may he regarded as consisting of earths, water, vegetable mould, decayed animal 

 substances, salts, ores, alkalies, gases, perhaps in a proportion corresponding to the order 

 in which they are now enumerated ; which is at any rate the fact with regard to the first 

 three, though their relative proportions are by no means uniform. The atmosphere has 

 been also found to consist of at least four species of elastic matter, nitrogen, oxygen, 

 carbonic acid gas, and vapour ; together with a multitude of minute particles detached 

 from the solid bodies occupying the surface of the earth, and wafted upon the wind.. 

 The two former ingredients exist in the proportion of about four to one ; carbonic acid 

 gas in the proportion of about one part in 100; and vapour in proportion still less. 

 Such then are the component principles of the soil and atmosphere, and the sources of vege- 

 table nourishment But the whole of the ingredients of the soil and atmosphere are not 

 taken up indiscriminately by the plant and converted into vegetable food, because plants 

 do not thrive indiscriminately in all varieties of soil. Part only ot the ingredients are 

 selected, and in certain proportions: as is evident from the analysis of the vegetable sub- 

 stance oiven in the foregoing chapter, in which it was found that carbon, hydrogen, 



