216 SCIENCE OF AGRICULTURE. Part II. 



If it be asked of what are the elementary organs themselves composed, the reply is, they 

 are composed, as it appears from the same analysis, of a fine, colourless, and transparent 

 membrane, in which the eye, aided by the assistance even of the best glasses, can discover 

 no traces whatever of organisation ; which membrane we must also regard as constituting 

 the ultimate and fundamental fabric of the elementary organs themselves, and, by conse- 

 quence, of the whole of the vegetable body. It has been asked by some phytologists 

 whether or not plants are furnished with vessels analogous to the blood-vessels of the 

 animal system. But if it be admitted that plants contain fluids in motion, which cannot 

 possibly be denied, it will follow, as an unavoidable consequence, that they are furnished 

 with vessels conducting or containing such fluids. If the stem of a plant of marigold is 

 divided by means of a transverse section, the divided extremities of the longitudinal fibres, 

 arranged in a circular row immediately within the bark, will be distinctly perceived, and 

 their tubular structure demonstrated by means of the orifices which they present, particu- 

 larly when the stem has begun to wither. Regarding it, therefore, as certain, that plants 

 are furnished with longitudinal tubes, as well as with cells or utricles for the purpose of 

 conveying or containing their alimentaiy juices, we proceed to the specific illustration of 

 both, together with tlieir peculiarities and appendages. 



1379. The utricles are the fine and membranous vessels constituting the cellular tissue of the pith and 

 pulp already described, whether of the plant, flower, or fruit. Individually they resemble oblong bladders 

 inflated in the middle, as in the case of some plants ; or circular or hexagonal cells, as in the case of 

 others. Collectively they have been compared to an assemblage of threads of contiguous bladders, or 

 vesicles, or to the bubbles that are found on the surface of liquor in a state of fermentation. 



1380. The tubes are the vessels formed by the cavities of the longitudinal fibres, whether as occurring in 

 the stem of herbaceous plants, or in the foot-stalk of the leaf and flower, or in the composition of 

 the cortical and ligneous layers, or by longitudinal openings pervading the pulp itself, as in the case of 

 the vine. 



1381. The larce tubes are tubes distinguishable by by being twisted from right to left, or from left to 

 the superior width of the diameter which they present 187 right, in the form of a corkscrew. They occur in 

 on the horizontal section of the several parts of the .^Ji^iasw. most abundance in herbaceous plants, particularly 

 plant. JS^^ SkS^ In aouatics. 



1382. Simple tubes (,f!g. 187.) are the largest of all Illlllmflll 1385. False spiral lubes aretubes apparently spiral 

 large tubes, and are formed of a thin and entire mem- Ullllfill PI ^ ^ slight inspection, but which, upon minute 

 brane, without any perceptible disruption of con- lllivlirillHl examination, are found to derive their appearance 

 tinuity. They are found chiefly in the bark, though lillll lllli merely from their being cut transversely by parallel 

 not confined to it, as they are to be met with also in Effilltlliliti fissures. 



Uie alburnum and matured wood, as well as in the lltiflMllFl ^^^^- ^^''^^'^ '"''f* a^^e tubes combining in one in- 



nbres of herbaceous plants. IllSililtllll dividual two or more^of the foregoing varieties. 



1383. Porou*<uAej resemble the simjile tubes in their illlliSfl 'n Mirbel exemplifies them in the case of the Biitomus 

 general aspect ; but differ from them in being pierced liill III HH umbellktus, in which the porous tubes, spiral tubes, 

 with small holes or pores, which are often distributed ] (|l| If |U I and false spiral tubes, are often to be met with united 

 In regular and jjarallel rows. They are found in lilRiff! P'f III in one. 



most abundance in woody plants, and particularly hi 1 iii' Ifrl I 1387. The small lubes are tubes composed of a suc- 



wood that is firm and compact, like that of the oak ; | PS|| fli ' ) cession of elongated cells united, like those of the 



but they do not, like the simple tubes, seem destined | [it '|| If ll cellular tissue. Individually they may be compared 



to contain any oily or resinous juice. ji III I P t it P ^o the stem of the grasses, which is formed of several 



1384. Spiral tuhtt are fine, transpatent, and thread- ^li|ll||||J" intemodia, separated by transverse diaphragms; and 

 like substances occasionally interspersed with the -*Jl*^ collectively to a united assemblage of parallel and 

 other tubes of the plant, but distinguished from them collateral reeds. 



' 1388. Pores are small and minute openings of various shapes and dimensions, that seem to be destined 

 to the absorption, transmission, or exaltation of fluids. They are distinguishable into perceptible pores 

 and imperceptible pores. 



1389. Gaps, according to Mirbel, are empty, but often regular and symmetrical, spaces formed in the 

 interior of the plant by means of a partial disruption of the membrane constitutmg the tubes or utricles. 

 In the leaves of herbaceous plants the gaps are often interrupted by transverse diaphragms formed of a 

 portion of the cellular tissue which still remains entire, as may be seen in the transparent structure of the 

 leaves of T^pha and many other plants. Transverse gaps are said to be observable also in the bark of some 

 plants, though very rarely. 



1390. There are various appendages connected with the elementary organs, such as internal glands, 

 internal pubescence, &c. : the latter occurs in dissecting the leaf or flower-stalk of JViiphar Ititea. 



Chap. III. 



Vegetable Chemistry, or Primary Principles of Plants. 



1391. As plants are not merely organised beings, but beings endowed with a species of 

 life, absorbing nourishment from the soil in which they grow, and assimilating it to their 

 own substance by means of the functions and operations of their different organs, it is 

 plain that no progress can be made in the explication of the phenomena of vegetable life, 

 and no distinct conception formed of the rationale of vegetation, without some specific 

 knowledge of the primary principles of vegetables, and of their mutual action upon one 

 another. The latter requisite presupposes a competent acquaintance with the elements 

 of chemistry ; and the former points out the necessity of a strict and scrupulous analysis 

 of the several compound ingredients constituting the fabric of the plant, or contained 

 within it. If the object of the experimenter is merely that of extracting such compound 

 ingredients as may be known to exist in the plant, the necessary apparatus is simple, 

 and the process easy : but if it be that of ascertaining the primary and radical principles 

 of which the compound ingredients are themselves composed, the apparatus is then 

 complicated, and the process extremely difficult, requiring much time and labour, and 



