DIV. I 



MORPHOLOGY 



usually occur. In all bundles the narrowest vessels are annular or 

 spiral ; the others are usually reticulated or pitted, but in the Pteri- 

 dophyta the elements, apart from the protoxylem, are scalariform 

 (Fig. 70 A). 



In the strands of phloem of the vascular bundles (Figs. 119, 120) 

 the sieve-tubes (v) which serve for the conduction of proteids are 

 always accompanied by other living cells. These are either the 



FIG. 119. Transverse section of a concentric bundle from the petiole of Pteris aquilina. sc, 

 scalariform vessels ; sp, protoxylem (spiral tracheides) ; sc*, part of a transverse wall showing 

 scalaiiform perforations ; Jp, xylem parenchyma ; v, sieve-tubes ; pr, protophloem ; pp, starch 

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



COMPANION CELLS (Fig. 120 s), which are usually shorter than the 

 elements of the sieve- tubes with which they connect by sieve-pits, 

 companion cells together with other elongated parenchymatous cells 

 (phloem parenchyma), or PHLOEM PARENCHYMA only (Fig. 119 s). 

 When the latter tissue is present the sieve-tubes are embedded in it 

 singly or in groups without intercellular spaces. 



Companion cells only occur in relation to the sieve-tubes of Angiosperms. 

 They are sister cells to the members of the sieve-tube, cut off by a longitudinal 



