106 



BOTANY 



phloem cease to perform their functions. The protoxylem elements 

 become compressed and ruptured by the tension resulting from the 

 continued vertical growth (a and a', Fig. 118), so that in their stead 

 a lysigenic intercellular space is often formed (Figs. 117, 118). The 

 protophloem elements (cf. Figs. 117, 118) at the same time become 

 disorganised, and their sieve-plates closed by a covering of callus. 



In accordance with the orientation of the xylem, the protoxylem 



Fig. 121.— Transverse section of the vascular-bundle cylinder of the petiole of Ptcris aquiline 

 sc t Scalariform vessels ; sp, protoxylem ; sc*, part of a transverse wall showing scalariform 

 perforations ; Ip, xylem parenchyma ; v, sieve-tubes ; pr, protophloem ; pjj, starch layer ; 

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



of roots is found on the outer, not on the inner side of the vascular 

 strands (Fig. 120). 



The Terminations of the Vascular Bundles. — In leaves, particu- 

 larly in the deciduous leaves of Angiosperms, the vascular bundles 

 become much branched until finally they are reduced to extremely 

 fine strands. In the leaves of Gymnosperms this branching of the 

 bundles does not usually take place, but instead, a single vascular 

 bundle frequently runs throughout the whole length of the leaf. The 



