THE FIBRO-VASCULAR BUNDLES. 



93 



imitating more or less the form of the whole. Exceptionally beautiful and instructive 

 skeletons of this nature are afforded by the stems of Tree-ferns, Dracaena, Yucca, 

 Maize, &c., when their parenchyma is perfectly destroyed by long-continued 

 decay, and only the epidermal tissue and the firm bundles in the interior remain. 

 The beginner would do well in any case to prepare for himself preparations of this 

 kind, or to examine them in collections ; they are, at least at first, extremely useful 

 for a right comprehension of their structure. This is, however, the case only with 

 lignified fibro-vascular bundles when they run isolated between soft parenchyma ; in 

 some plants, on the contrary, the tissue of the bundles is even softer and more deli- 

 cate than that of their environment {e. g. Ceratophyllum, Myriophyllum, Hydrilleoe, 

 and other water-plants) ; in these cases they cannot of course be isolated. But in 

 the older Hgnified stems and roots of Conifers and Dicotyledons, the fibro-vascular 

 bundles are so densely crowded, and so developed by further tissue-formation, that 

 at last very little or even nothing is left of the original fundamental tissue which 

 separated them, and such stems consist almost entirely of fibro-vascular masses. 



Each separate fibro-vascular bundle consists, when it is sufficiently developed, of 

 several different forms of tissue, and must therefore itself be considered as a tissue- 

 system ; but different bundles, often in very large number, unite in most plants to 

 form a system of a higher order. At present however we shall consider only the 

 separate bundle. 



The fibro-vascular bundle consists at first of similar cells combined without 

 intercellular spaces^- this form of tissue of the young bundle, which has not yet 

 undergone differentiation, may be termed Procamhium ^. As it grows older, 

 single cells of the rows forming the young bundle change into permanent cells of 

 definite form (vessels, bast, &c.) ; from these points of origin the transformation 

 of the procambium-cclls into permanent cells in the transverse section of the bundle 

 advances until the cells are altogether changed into permanent cells ; or an 

 inner layer of the bundle remains in a condition capable of further development, 

 and is then called Cambium. In advanced age there are thus bundles devoid of 

 and bundles containing cambium ; the former may be termed closed, the latter open ^. 

 As soon as a procambium bundle has become transformed into a closed fibro- 

 vascular bundle, all further growth ceases, as in Cryptogams, Monocotyledons, and 

 some Dicotyledons. The open fibro-vascular bundle, on the other hand, continues 

 to produce new layers of permanent tissue on both sides of its cambium, and thus 

 the portion of the stem or root concerned continually increases in thickness, as 

 occurs in woody Dicotyledons and Conifers ; the leaf-structures, however, of these 



^ The young cells of the fibro-vascular masses are not always elongated and prosenchymatous ; 

 in the roots, e.g. oi Zea Mais, the young tissue-cells which no longer divide and their neighbours are 

 diagonally tabular or cubical. 



^ Nageli calls the tissue of the young fibro-vascular bundles simply Cambium, and distinguishes 

 by the same term the tissue, capable of further development, of the bundles which increase in thick- 

 ness, which nevertheless ought to be distinguished from them. — Sanio terms the latter only Cam- 

 bium, which I adopt. (Sanio in Bot. Zeitg. p. 362, 1863.) 



^ This distinction was first made by Schleiden, but he incorrectly ascribed to Dicotyledons in 

 general only open bundles ; his distinction of simultaneous and successive cannot be sustained ; all 

 bundles become differentiated successively in transverse section. Sclileiden's simultaneous bundles 

 of the higher Cryptogams belong to the closed description. 



