. Un manuel de botanique : être une introduction à l'étude de la structure, la physiologie et la classification des végétaux . La botanique. 22 FONCTIONS DES NAVIRES. produites dans les plantes vasculaires. À l'allongement des cellules, et le dépôt de couches d'épaississement et de fibres dans leur intérieur, les différents navires doivent leur origine. Ainsi, lorsque les cellules sont allongées en forme de fusée, comme les tubes, et leurs parois sont épaissies et durci par depo- sitions de matière ligneuse, elles donnent lieu à un Pleurenchyma ; et lorsque les tubes membraneux allongées sont renforcés par des fibres en spirale, l'difierent types de tissu fibro-vasculaires

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. A Manual of botany : being an introduction to the study of the structure, physiology, and classification of plants . Botany. 22 FUNCTIONS OF VESSELS. produced in vascular plants. To the elongation of cells, and the deposition of thickening layers and fibres in their interior, the various vessels owe their origin. Thus when cells are elongated, as spindle- shaped tubes, and their walls are thickened and hardened by depo- sitions of ligneous matter, they give rise to Pleurenchyma; and when elongated membranous tubes are strengthened by spiral fibres, the difierent kinds of Fibro-vascular tissue are produced. The spiral vessel may be considered as the type of the last-mentioned tissue, and aU its varieties may be traced to difierent conditions in de- velopment of the fibre. In the case of some vessels, their forma- tion can be distinctly traced to cells placed end to end, the partitions between which have been ab- sorbed. The raoniliform or beaded appearance often presented by the different kinds of vessels, more espe- cially the Pitted, plainly indicates this mode of for- mation. Occasionally cellular prolongations are seen in the interior of pitted vessels, giving rise to what has been called Tylosis (ruXos, swelling or protru- sion). It has been noticed in the vessels of Oak, Kg- n. Chestnut, Wahiut (fig. 71 a), Ash, Elm, etc. As in cells, so in vessels, the walls are composed of cellulose, and there are usually no visible perforations ; the communication between them taking place by imbibition or osmose. In some instances, when vessels are closely applied to each other, especially when they overlap, the membrane becomes absorbed, and direct communication takes place. This has been seen in spiral and pitted vessels. The pits or depressions on the walls of vessels, and the thinning of the tissue at particular points, appear to serve the purpose of allowing the rapid transmission of fluids. Pleurenchyma, in its early state, contains fluids, and conveys them from one

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• bookcentury1800
• bookdecade1870
• booksubjectbotany
• bookyear1875