33^ 



PRT\fARy ARRAyGF^(E^'T OF TISSUES. 



the scale-leaves, ihey assume a changed structure, which they maintain throughout 

 their whole course in the leaf. In the round cross-section of the bundle (Figs. 158, 159), 

 a small group of narrow spiral tracheides {s/>) (the primitive elements of the xylem) 

 occupies about the centre. From this an uninterrupted group of large prismatic pitted 

 tracheides (the inner portion o£ the xylem) extends towards the inside ; this group 

 occupies the entire inner side of the bundle, and has in cross-section the form of a 



FIG. 158.— Cycas revoluta. Petiole of a small foliagc-lcaf belonging to a young plant ; vascular bundle, cross-section 

 (225). Explanation in the text The spiral tracheide s^ is connected with the inner pitted tracheides t by means of a 

 V^oup of annular and reticulated tracheides. Jn c here and there fragments of the large crystals of Calcium oxalate. 



sector of a circle with the centre at the primitive tracheides. The remaining part of 

 the bundle, lying outside that described, is principally formed of thin-walled elements, 

 ranged in radial rows : next the outer edge are several concentric rows of sieve-tubes 

 {s), separated by delicate parenchyma ; the outermost row which bounds the bundle, 

 being in the mature condition compressed in the manner often described, and thicker- 

 walled than the rest, bordering the outer edge as a narrow shining band (/>) ; on the 

 boundary of the xylem, so far as present investigations extend, only prismatic cells 

 occur, without sieve-tubes. To these parts is finally added an external portion of 

 the xylem, developed in centrifugal order, outside the primitive tracheides ; this forms 

 a small group of pitted tracheides (a) ranged in irregular radial rows, which are 

 separated from one another, from the primitive tracheides, and from the inner 

 portion of the xylem, by thin-walled elements. As the bundles become thinner in 



I s 



