326 SCIENCE OF AGRICULTURE. Fawc II. 



it doubtful whether they are in reality vegetable productions or no. The same thing may be said of some 

 of the other ingredients that have been found In the ashes of plants, which it is probable they have ab- 

 orbed ready formed by the root, and deposited unaltered, bo that they can scarcely be at all regarded as 

 being the genuine products of vegetation. 



1483. Other substances. Besides the substances above enumerated, there are also several others that have 

 been supposed to constitute distinct and peculiar genera of vegetable productions, and which might have 

 been introduced under such a character ; such as the mucus, jelly, sarcocol, asparagin, inulin, and ulmin, 

 ^Qf Dr. Thomson, as described in his well known Si/stem of Chetnistry ; but as there seems to be some dif- 

 ference of opinion among chemists with regard to them, and a belief entertained that they are but va- 

 rieties of one or other of the foregoing ingredients, it is sufficient for the purposes of this work to have 

 iperely mentioned their names. Several other substances of a distinct and i)eculiar character have been 

 suspected to exist in vegetable productions : such as the febrifuge principle of Seguin, as discovering itself 

 in Peruvian bark ; the principle of causticity or acridity of Senebier, as discovering itself in the roots of 

 ranunculus bulbosus, scilla maritima, bryonia alba, and arum maculatum, in the leaves of digitalis pur- 

 purea, in the bark of daphne mezereon, and in the juice of the spurges, to which may be added the fluid 

 secreted from the sting of the common nettle, the poisons inherent in some plants, and the medical virtues 

 inherent in others ; together with such peculiar principles as may be presumed to exist in such regions of 

 the vegetable kingdom as remain yet unexplored. The important discoveries which have already re- 

 sulted from the chemical analysis of vegetable substances encourage the hope that further discoveries will 

 be the result of further experiment ; and from the zeal and ability of such chemists as are now directing 

 their attention to the subject, every thing is to be expected. 



Sect. II. Simple Products. 



1484. A very few constituent and uncompouhded elements include all the compound in- 

 gredients of vegetables. The most essential of such compounds consist of carbon, 

 oxygen, and hydrogen ; a small proportion of nitrogen is said to be found only in cru- 

 ciform plants. The remaining elementary principles which plants have been found to 

 contain, although they may be necessary in the vegetable economy, yet they are by no 

 means principles of the first importance, as occurring only in small proportions, and 

 being dependent in a great measure on soil and situation ; whereas the elements of car- 

 bon, oxygen, and hydrogen, form as it were the very essence of the vegetable subject, 

 and constitute by their modifications the peculiar character of the properties of the plant. 

 This is conspicuously exemplified in the result of the investigations of Gay Lussac, and 

 Thenard, who have deduced from a series of the most minute and delicate experiments 

 the three following propositions, which they have dignified by the name of Laws of Ve- 

 getable Nature (Traitd de Cliem. Element, tom. iii. chap, iii.) : 1st, Vegetable sub- 

 stances are always acid when the oxygen they contain is to the hydrogen in a greater 

 proportion than in water ; 2dly, Vegetable substances are always resinous, or oily, or 

 spirituous, when the oxygen they contain is to the hydrogen in a smaller proportion than 

 in water ; Sdly, Vegetable substances are neither acid nor resinous, but saccharine, or 

 mucilaginous, or analogous to woody fibre or starch, when the oxygen and hydrogen they 

 contain are in the same proportion as in water. (See Dr. Thomson s System of Chemistry.) 



Chap. IV. 

 Functions of Vegetables^ 



1485. The life, growth, and propagation of plants necessarily involves the several 

 following topics : germination, nutriment, digestion, growth and developement of parts, 

 anomalies of vegetable developement, sexuality of vegetables, impregnation of the 

 vegetable germen, changes consequent upon impregnation, propagation and dispersion 

 of the species, causes limiting the dispersion of the species, evidence and character of 

 vegetable vitality. 



Sect. I. Germination of the Seed. 



1486. Germination is that act or operation of the vegetative principle by which the 

 embryo is extricated from its envelopes, and converted into a plant. This is univer- 

 sally the first part of the process of vegetation. For it may be regarded as an indu- 

 bitable fact, that all plants spring originally from seed. The conditions necessary to 

 germination relate either to the internal state of the seed itself, or to the circumstances 

 in which it is placed, with regard to surrounding substances. 



1487. The first condition necessary to germination is, that the seed must have reached maturity. Un- 

 ripe seeds seldom germinate, because their parts are not yet prepared to form their chemical combinations 

 on which germination depends. There are some seeds, however, whose germination is said to commence 

 in the very seed-vessel, even before the fruit is ripe, and while it is yet attached to the parent plant. 

 Such are those of the tangekolli of Adanson, and agave vivipara of East Florida, as well as of the cyamus 

 nelumboof Sir J. E. Smith, or sacred bean of India; to which may be added the seeds of the common 

 garden-radish, pea, lemon, &c. But these are examples of rare occurrence ; though it is sometimes 

 necessary to sow or plant the seed almost as soon as it is fully ripe, as in the case of the coffee-bean ; 

 which will not germinate unless it is sown within five or six weeks after it has been gathered. But 

 most seeds, if guarded from external injury, will retain their germinating faculty for a period of many 

 years. This has been proved by the experiment of sowing seeds that have been long so kept ; as well as 

 by the deep ploughing up of fields that have been long left without cultivation. A field that was thus 

 ploughed up near Bunkeld, In Scotland, after a period of forty years' rest, yieltled a considerable bl^de of 



