288 



STRUCTURE OF STEMS AND ROOTS 



m 



457. Cross-section of fibro-vas- 

 cular bundle of moonseed (see 

 Fig. 455) . /, /, crescent-shaped 

 sheaths of bast fiber; p, 

 phloem; cp, crushed phloem; 

 c, cambium; d, xylem ducts; t, 

 xylem tracheids; m, medullary 

 rays of fundamental tissue; 

 from c to / (at bottom) , xylem ; 



1, end of first year's growth; 



2, end of second year's growth 

 of wood. 



mon arrangement of the bundles is 

 in the form of a circle some distance 

 from the center, with a few other 

 bundles within the circle. Within 

 the circle also are sometimes large 

 areas of fibrous tissue. (Fig. 456.) 

 There are, however, wide variations 

 from this structure, but this mode 

 of arrangement is often called the 

 fern type of stem. 



489. Three Types of Bundles. 

 It has already been said (481) that 

 every fibre-vascular bundle is made 

 up of two parts- (1) phloem or 

 soft bast; (2) xylem' or woo(}. The 

 relative position of these two 

 strands of tissue is very important. 

 There are three plans of arrange- 

 ment, on which three types of 

 These plans are collateral, bi-collateral 



bundles are based, 

 and concentric. 



490. In collateral bundles, the phloem and xylem are 

 placed side by side, the 

 xylem being nearer the 

 center of the stem and 

 the phloem outside or 

 nearer the circumfer- 

 ence of the stem. This 

 plan occurs in the stems 

 of phenogams. The col- 

 lateral bundles may be 

 either open or closed. 

 Open bundles are those 

 that continue to in- 



, . ,. 458. Part of cross-section of root-stock of aspar- 

 in Size during lite agus , showing a few fibre-vascular bundles. 



