GENERAL CHARACTERS OF XYLEM AND PHLOEM. 



33 



follows only late in the thick-walled fibres — the proper bast-cells: these, however, 

 often also remain entirely unlignified. The pliability of true bast, e. g. of Flax- and 

 Hemp-fibres, simply depends upon their being not lignified at all, or only to a very 

 slight extent. The other elements of the phloem are remarkable for their soft, pure 

 cellulose walls. Diflicult though it is, however, to separate phloem and xylem 

 sharply from one another by words, the difference of their aspect under the 

 microscope is nevertheless usually conspicuous. It is especially characteristic for 



Fig. 138.— Part of a transverse section of the completely extended hypocotyledonous portion of the stem of 

 Ricinus communis, r parenchyma of the primary cortex, and m pith, both belonging to the fundamental tissue. 

 Between r and b is the simple vascular-bundle sheath (containing starch), which also belongs to the fundamental 

 tissue. The bundle consists of phloem {b,y), xylem (g, t), and cambium [c. c), and is therefore an open bundle. The 

 cambium (c, <-) extends into the fundamental tissue situated between the bundles, as interfascicular cambium (f, *), 

 which arises by subsequent division of the large parenchyma cells, b, b bast fibres ; y, y soft bast (partly paren- 

 chyma, partly sieve-tubes) ; t, t narrow, and g, g wide, dotted vessels, with wood prosenchyma between. 



the xylem, that at least some air-conducting vessels are present; while the 

 vascular elements of the phloem, the sieve-tubes, are filled with slime containing 

 albuminous matters, or clear sap, and never contain air. So far as we have 

 yet succeeded in obtaining an insight into the physiological significance of the 

 vascular bundles, the xylem appears to be the tissue which provides chiefly for the 

 conveyance of the water taken up by the roots, and the mineral nutritive matters 

 contained in it, to the organs of assimilation: however the true function of the 

 cavities of the vessels, which, as we shall see later, contain very rarified air, and 



