MORPHOLOGY 



105 



are encircled by phloem consisting of sieve -tubes (v) and phloem 

 parenchyma (s). 



Such vascular bundle strands occur in the Ferns and Selagincllaccac, and also 

 in the Lycopodiaceac, where they exhibit even a greater degree of coalescence. 

 In the Equisetaeeae the vascular bundles correspond more nearly to the collateral 

 bundles of the Phanerogams. 



The vascular bundles are developed from strands of meristematic 

 tissue which are called pro- 



CAMBIUM STRANDS. In those 



portions of plants which still 

 retain an energetic vertical 

 growth, the procambium strands 

 remain undifferentiated, except 

 at definite points, where single 

 rows of cells lose their meri- 

 stematic condition and form 

 narrow, annular, and spiral 

 vessels and sieve-tubes, or sieve- 

 tubes and companion cells, the 

 structure of all of which is of 

 such a nature as to render 

 their elongation possible. Such 

 primary vascular elements are 

 termed protoxylem ; while the 

 corresponding sieve elements 

 are in like manner designated 

 protophloem. The protoxylem 

 occupies the innermost, the pro- 

 tophloem the outermost side of a procambium strand, from which a 

 collateral bundle is eventually formed. After the vertical growth of 

 any part of a plant ceases, the differentiation of the procambium 

 strand into a collateral vascular bundle is continued from the inner 

 and outer sides of the strand toward the centre. If the whole 

 meristematic tissue of a procambium strand is exhausted in this 

 process, the finally-developed vascular bundles are said to be closed ; 

 but if any of the meristematic tissue remains in an undifferentiated 

 condition between the xylem and phloem portions, the bundles are 

 spoken of as OPEN. The Pteridophytes have, almost without excep- 

 tion, closed bundles ; in Monocotyledons also the bundles are always 

 closed (Fig. 117); Gymnosperms and Dicotyledons, on the contrary, 

 have open bundles (Fig. 119). 



The meristematic tissue which remains undifferentiated between 

 the xylem and phloem portions of a bundle is called the CAMBIUM 

 (Fig. 119). 



In fully -developed vascular bundles the protoxylem and proto- 



Fio. 120.— Transverse section of central portion of 

 the root of Acorus Calamus, m, Medulla ; s, 

 xylem ; v, phloem ; p, pericycle ; e, endodermis ; 

 c, cortex, (x 90.) 



