146 



SCIENCE OF GARDENING. 



Part II. 



Sect. III. Elementary or Vascular Organs. 

 627. From the previous analysis of the composite organs it appears they are all ulti- 

 mately reducible to fibres, cellular tissue with or without parenchyma, and reticulated mem- 

 brane, which we must consequently regard as being, under one modification or other, the 

 ultimate and elementary organs of which the whole mass of the plant is composed. If it 

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

 composed, as appears from the same analysis, of a fine, colorless, and transparent mem- 

 brane, 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 is 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. The same sort of structure may be observed 

 in the stem of cucurbitaceous plants also, particularly in that of the gourd, in which there 

 are besides discoverable several sets of longitudinal tubes situated near the centre, and 

 of considerable diameter. 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 contain- 

 ino°their alimentary juices, we proceed to the specific illustration of both, together with 

 their peculiarities and appendages. 



628 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 in- 

 flated 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. 



629 The tubes are the vessels formed by the cavities of the longitudinal fibres, whether as occurring m 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 They 

 have generally been characterised under the denominations of proper vessels, lymphatics, and trachea?. 

 But as this is rather a premature reference to their different uses, which is besides not altogether correct, 

 we shall adopt, with a little alteration, the denominations introduced by Mirbel, as arising from fheir 

 form or structure. The first and primary division founded upon this principle is that by which they are 

 distributed into large tubes and small tubes. .-«_-*«_.«■ t u^ 



630. The large tubes are tubes distinguishable by the superior width of the diameter which they present on 

 the horizontal section of the several parts of the plant 



Simple tubes ( fig-. 52.) are the largest of all the large 

 tubes, and are formed of a thin and entire membrane, 

 without any percemible disruption of continuity, and 

 are found chiefly in the bark, though not confined to 

 it, as they are to be met with also in the alburnum 

 and matured wood, as well as in the fibres of herb- 

 aceous plants. . 



Porou* tubes resemble the simple tubes in their 

 general aspect ; but differ from thern in being pierced 

 with small holes or pores, which are often distributed 

 in regular and parallel rows. They are found in 

 most abundance in woody plants, and particularly in 

 wood that is firm and compact, like that of the oak ; 

 but they do not, like the simple tubes, seem destined 

 to conta'in am- oily or resinous juice. 



Spiral tubes are fine, transparent, and thread- 



like substances, occasionally interspersed with the 

 other tubes of the plant, Dut distinguished from them 

 by being twisted from right to left, or from left to 

 right, in the form of a corkscrew. They occur in 

 most abundance in herbaceous plants, particularly 

 in aquatics. 



False spiral tabes are tubes apparently spiral on a 

 slight inspection, but which, upon minute examine 

 ation, are found to derive their appearance merely 

 from their being cut transversely by parallel fissures. 



Mixed tubes are tubes combining in one individual 

 two or more of the foregoing varieties. Jtirbel exem- 

 plifies them in the case of the butomus umbellatus, 

 in which the porous tubes, spiral tubes, and false 

 spiral tubes, are often to be met with united in one. 



631. The small tubes are tubes composed of a succession of elongated cells united, 

 like those of the cellular tissue. Individually they may be compared to the stem of the 

 grasses, which is formed of several internodia, separated by transverse diaphragms ; and 

 collectively to a united assemblage of parallel and collateral reeds. 



632 Pores are small and minute openings of various shapes and dimensions, that seem to be destined to the 

 absorption, transmission, or exhalation of fluids. They are distinguishable into the following two sorts : 

 perceptible pores and imperceptible pores. The perceptible pores are either external or internal, and are 

 the apertures described by Hedwig as discoverable in the network constituting the epidermis. The /??/- 

 perceptiile pores are pores that are not distinguishable by the eye, unless assisted with the best glasses ; 

 but they are known to exist by the evidence of experiment, and have lately been ably delineated and de- 

 scribed by A. T. Thomson, in his Lectures on Botany. (Vol. i. p. 609.) 



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

 terior of the plant by means of a partial disruption of the membrane constituting 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 typha and many other plants. Transverse gaps are said to be observable also in the bark of some 

 plants, though very rarely. ..,.,, j - » , 



634 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 nympheea lutea. 



