SECT. I 



MORPHOLOGY 



125 



vessels, and sieve-tubes, or sieve-tubes and companion cells ; tlie structure of all of 

 these is of such a nature as to render their elongation possible. Such primary 

 vascular elements are termed photoxylem ; while the corresponding sieve elements 

 are in like manner designated protophlokm. The protoxylem occupies the inner- 

 most, the protophloem the outermost side of a procanibium strand, from which a 

 collateral bundle is eventually formed. After the growth in length of any part of 

 a plant ceases, the differentiation of the procanibium strand into a collateral vascular 



Fig. 135. — Transverse section of a concentric bundle from the petiole of Pteris aqvilina. fc, 

 Scalariform vessels ; sp, protoxylem (spiral traclieides) ; sr*, part of a transverse wall showing 

 scalariform perforations ; Ip, xylem parenchyma ; v, sieve-tubes ; pi; protophloem ; pp, starch 

 layer ; e, endodermis ; s, phloem parenchyma, (x 240.) 



bundle is continued from the inner and outer sides of the strand toward the 

 centre. 



In fully developed vascular bundles the proto.xylem and in'otoi)hloeni cease to 

 perform their functions. The protoxylem elements become compressed and ruptured 

 by the tension resulting from the continued vertical growth {a and «', Fig. 132), 

 so that in their stead a lysigenic intercellular space is often formed (Figs. 131, 132). 

 The protophloem elements at the same time become disorganised, and their sieve- 

 plates closed bj' a covering of callus. 



In accordance with tlie inverted orientation of the xylem, the protoxylem of 

 roots is found on the outer, not or. the inner side of the vascular strands (Fig. 134). 



