THE FIBRO-VASCULAR BUNDLES. 



f3f 



section of the vascular bundle often presents a confusing chaos of lines (contours of 

 the cell-walls). In order to obtain a clear insight into the microscopic structure of 

 the bundles, it is necessary carefully to compare transverse and longitudinal sections ; 

 since the characteristic forms of the cells are generally visible only in the lateral 

 view (and therefore in the longitudinal section), while the arrangement and grouping 

 of the elements are more evident in the transverse section. In addition, however, 

 it happens that it is just in the study of the vascular bundles that the relations 

 of symmetry prevailing in the anatomical structure of the plant, which we shall 

 bring under general consideration later on, are particularly conspicuous ; so that a 

 longitudinal section through a bundle, carried in the radial direction from the 

 outside inwards, affords a view quite different from that obtained when the knife 

 meets the bundles parallel to the surface of the organ, or in any other direction. As 



FIG. 136.— Transverse section of a feebly developed • 

 Fern), s phloem ; sp narrow spiral tracheides— the v 

 cndodermis. (After De Eary.) 



ciliar bundle from the rhizome of Polypodium vulgare 

 er lumina belong to the broad scalariform tracheides. 



a guide to the structure of a vascular bundle, always somewhat complicated, the dis- 

 tinction (first given by De Bary^) of the total mass of its cells into two subdivisions, 

 the Bast-portion {Phloem of Nägeli) and the Wood-portion {Xylan of Nageli), is 

 especially serviceable. Both groups consist in general of elongated, often very 

 long, vesicular or tubular cells, narrow, or very narrow in transverse section ; so 

 that on a transverse section through an organ the vascular bundles usually come 

 into view at once as groups of cells with peculiarly narrow lumina, situated in the 

 large-celled parenchyma of the fundamental tissue. Individual elements, especially 



* De liary, I.e. p. 330. I here bring forward De Bary's subdivision of the tissue of the vascular 

 bundles into Gcfiissthcil and Sicbthcil — the earlier subdivision into Wood and Bast {Xylon and 

 Phloem), — because by Schwendener's misusage of the word 'Bast,' by which he distinguishes all 

 possible sclerenchymatous tissues not belonging to the vascular bundles, a confusion pernicious 

 to the beginner has been introduced into the good old nomenclature. 



