T I S 



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T I S 



enlarge, and then transmit a little light, which, on account 

 of their minute dimensions, is not suffered to pass as a 

 white pencil, but is decomposed in its course, the granule 

 thereby becoming of a greenish hue. The granules ex- 

 hibiting this greenish hue are now in a fit state to enter 

 into the composition of the fibre that is to exist in the in- 

 terior of the membranous tube.' This is effected in the 

 following manner : ' The granules which are in active 

 motion in the viscid fluid near one of the ends become 

 severally attracted to the inner wall of the vessel, begin- 

 ning at the very point ; those granules first attracted ap- 

 pear as if cemented to the spot by the viscid fluid in that 

 direction losing some of its watery character; for there 

 appears a string of a whitish colour, besides granules, in the 

 line which the fibre is to occupy. As the other granules 

 are attracted to those already fixed in an inclined direction, 

 the spiral course is soon to be seen, and the same action 

 progressively goes on from the end where it began towards 

 the other, around the interior of the tube in the form of a 

 spiral ; the fibre being produced, like a root, by having the 

 new matter added and continually attached to the grow- 

 ing point, thereby causing its gradual elongation.' Spiral 

 cells and vessels thus formed exhibit a great variety of 

 appearance, depending on the period in the age of the 

 cell or tube at which the development of the fibre takei 

 place, as well as the modifications that occur in the 

 chemical changes of the substances from which the fibres 

 are formed. The cause of the arrangement of the par- 

 ticles in a fibrous form is still got satisfactorily explained, 

 and it is most commonly referred to an attraction between 

 the sides of the membrane, of the cell, and the particles it 

 contains, but why they form a spiral is a mystery yet to be 

 solved. [SPIRAL STRUCTURE OF PLANTS.] 



The various forms of vegetable tissue found in the dif- 

 ferent organs of plants are included in the following ar- 

 rangement : 



I. FIBROUS. 



Tissue in which elementary fibre is alone apparent. 



II. CELLULAR. 



Tissue composed of membrane in the form of cells whose 

 length does not greatly exceed their breadth. 



1. Merenchyma, the cells of which touch each other only 

 at some points. 



2. Parenchyma, the walls of the cells of which are ge- 

 nerally united. 



3. Prosenchyma, the cells of which are always fusiform, 

 and overlie each other at their ends. 



III. VASCULAR. 



Tissue composed of cylindrical tubes of membrane con- 

 tinuous, or overlying each other at their ends. 



1. Pleurenchyma, with the sides of the tubes thickened 

 and tapering to each end. 



2. Cinenchyma, the sides of the tubes of which anasto- 

 inoze, and convey a peculiar fluid. 



IV. FIBRO-CELI.ULAR. 



Tissue composed of cells, in the inside of which fibres 

 are generated. 



a. Genuine. 



1. Fibrous cells. 



b. Spurious. 



2. Porous cells. 



3. Dotted cells. 



V. FlBRO-VASCULAR. 



Tissue composed of tubes, in the inside of which one or 

 more spiral fibres are more or less perfectly developed. 



a. Genuine. 



1. Spiral vessels. 



2. Annular vessels. 



3. Moniliform vessels. 



b. Spurious. 



4. Scalariform vessels. 



5. Porous vessels. 



(Bothrenchyma.) 

 G. Dotted vessels. 



This arrangement includes the principal forms of tissue 

 observed in plants, but the divisions are not founded upon 

 any pssrutiM difference in the structure or functi 



. The most important distinction exists 



I between membrane and fibre, which arc apparently deve- 

 loped under the influence of different forces. The cell and 

 the tube differ but in their dimensions, and the same is 

 true of them when fibre is generated in their inside. 



Fibrous Tissue. Although the development of fibre in- 

 dependent of membrane is still undecided, many of the 

 parts of plants exhibit fibres divested of membrane. Fibres 

 spirally arranged and adhering only together by vegetable 

 mucus, which is dissolved away by the application of water, 

 were discovered by Brown, in the seed-coat of Casuarina, 

 and by Lindley, in the same position in Collomia iincaris. 

 Meyen, who maintains that all cells may be composed of 

 minute fibres, records many instances of vegetable struc- 

 ture in which fibre of a spiral form alone is most apparent, 

 as the parenchyma of a species of Stelis, in the external 

 layer or bark of the aerial roots of many species of Orchi- 

 daceae, and also in species of Melocactus and Mammillaria. 

 Fibres not assuming a spiral form, and independent of 

 cells or tubes, have been described by Purkinje. In the 

 lining of the anthers of Polygala Chamsebuxus they are 

 found short, straight, and radiating ; in the anthers of Li- 

 naria cymbalaria they form distinct arches ; and in those 

 of some species of Campanula, they are arranged like the 

 teeth of a comb. The fibre in all cases is very minute, 

 varying from ^ to -^ of an inch in diameter. It is most 

 commonly transparent and colourless, but in some cases 

 has been observed of a greenish colour. Purkinje, who 

 has recently investigated it very attentively, asserts that it 

 is hollow ; but Lindley, Schleiden, and Morren are of 

 opinion that it is solid. 



Cellular Tissue; also called utricular and vesicular tissue 

 the Parenchyma of Lindley and Morren, tela cellulosa ot 

 Link-, and contextus and coinplexus cellulosus of older 

 writers ; Zellgewebe, Germ. ; TuiU cellulaire, French. 

 This tissue consists of cells or cavities, which are closed on 

 all sides, and are formed of a delicate, mostly transparent 

 membrane developed from a cytoblast. It is present in the 

 whole vegetable kingdom ; and all the lower forms of plants, 

 constituting the class Acrogens, are composed entirely of it, 

 and have hence been called Cellulares. In the higher 

 plants it is most abundant in fruits and succulent leaves. 

 It exists in larger quantity in herbs than trees, and the 

 younger the plant is the more it abounds, and constitutes 

 the entire structure of the embryo. 



The normal form of the cells is spheroidal, and when 

 they exist in this or in an elliptical form, and only touch 

 each other at a few points without exerting pressure, they 

 constitute the tissue called by Meyen Merenchyma. The 

 cells in this case may form a regular or irregular layer, a 

 distinction which may be of some importance. Such tissue 

 is found in many parts of plants, especially those which 

 are delicate and easily torn, as in the pulp of fruits like the 

 strawberry, in the petals of the white lily, in the stem of 

 Cactus pendulus, where they are spheroidal, and in the 

 leaf of the Agave Americana, where they are elliptical. 

 The cells also which constitute the entire of many of the 

 lower plants belong to this division of cellular tissue. 

 They are seen separate or loosely adhering to each other 

 in the Protocqccus nivalis, the plant of the Red snow 

 [SNOW, RED], in many of the smuts and brands, as Ustilago 

 and Uredo. Chroolepus, and many of the lower forms of 

 alga: and fungi, consist of filaments which are entirely 

 composed of spheroidal cells arranged one upon another. 



In the higher forms of plants the vegetative force is 

 greater, and a greater number of cells being generated in 

 a given space, they press on each other on all sides, 

 assuming a variety of forms, and constituting the tissue 

 called by Meyen Parenchyma. The most common form 

 which the cells present under these circumstances is the 

 rhomboidal dodecahedron, which is the mathematical form 

 that a globe assumes when subjected to the pressure of a 

 number of globes touching each other at the same time. 

 These cells when cut through, as in the section of a por- 

 tion of pith, or the leaf of a plant, will present their cut 

 margins, when seen through the microscope, in the form 

 of hexagons, (a and b. Fig. 1.) But the pressure is not 

 always equal on all sides of the cells, so that a great mir.i- 

 ber of secondary forms are the result. When the vesicles 

 are elongated, the dodecahedrons assume the form of right- 

 angled prisms, terminated by four-sided pyramids, whose 

 faces replace the angle of the pyramids at varying degrrcs 

 of inclination to the axis. Many of the forms thus iisMimed 

 characterise parts of plants, and are very constant in the 



