33] CHAPTER II. THE TISSUES. 121 



33. The Primary Vascular Tissue. The primary vas- 

 cular tissue is differentiated from the procambium of the stele in 

 the form of strands or bundles, vascular bundles. The vascular 

 tissues of the bundles are either tracheal tissue (p. 93), which is 

 always lignified, and is termed icood or xylem ; or sieve-tissue 

 (p. 94), which is termed bast or phloem. A vascular bundle may 

 consist, either exclusively of wood or of bast ; or of both wood and 

 bast, when it is said to be a conjoint bundle. It is generally the 

 case that a varying proportion of sclerenchyma (stereom) is associ- 

 ated with the vascular tissue; hence the bundles are frequently 

 spoken of as fibro-vascular bundles. As a rule, an equal number 

 of wood-bundles and of bast-bundles are differentiated in a stele, 

 whether they be isolated or conjoined ; there may be only one of 

 each (e.g. finer branches of the dichotomous roots of most Lycopo- 

 diums) or there may be a very considerable number (e.g. stems of 

 Monocotyledons). 



With regard to the occurrence of vascular tissue in the gametophyte- 

 generation, and in the sporophyte of the lower plants, it may be stated 

 that lignified vascular tissue (i.e. wood) does not occur in any gameto- 

 phyte, nor in the sporophyte of any plant below the Pteridophyta. How- 

 ever, in the stem of the gametophytic shoots of some Mosses there is a 

 solid central stele consisting of tissue which is functionally vascular 

 tissue; the same is true of the stem (seta) of the Moss-sporophyte in 

 certain cases. Sieve-tissue has been found in some of the larger Brown 

 Seaweeds (p. 96). 



The primary vascular tissue-system extends continuously 

 throughout the body of the sporophyte of the higher plants ; the 

 vascular bundles of root, stem, and leaf are all in direct com- 

 munication. 



The arrangement and course of the vascular bundles are in- 

 timately connected with the morphology of the plant and with the 

 differentiation of its members. In elongated members (stems, 

 petioles, roots) the bundles run longitudinally, so that a transverse 

 section of such a member shows transverse sections of its vascular 

 bundles. 



In the primary root the longitudinal course of the bundles is 

 simple ; there is an axial vascular cylinder, either solid or hollow, 

 consisting of straight. more or less distinct, bundles of wood and 

 bast, and extending from the growing-point backwards to where 

 the root merges into the stem ; from this cylinder there arise 



