CONDUCTING TISSUES 129 



of bundles are recognized. Radial fibro-vascular 

 bundles are always incomplete,, consisting of xylem 

 or phloem. They are further characterized by the 

 arrangement of xylem or phloem elements in a circle 

 within the endodermis, and the alternation of each 

 xylem bundle with a phloem bundle. Radial bundles 

 are found in all young roots and may even be present 

 in mature monocotyledonous roots. They are an 

 indication of primary structure in those plant 

 parts in which they occur. Concentric fibro-vascular 

 bundles are complete bundles consisting of xylem 

 and phloem. They are characterized by the arrange- 

 ment of xylem and phloem elements in such a way 

 that one of these surrounds the other. Most fre- 

 quently the xylem surrounds the phloem, and the 

 bundles are scattered irregularly in a central pith 

 region within the endodermis. This type of bundle 

 occurs only in monocotyledonous roots and stems. 

 Collateral fibro-vascular bundles are of the complete 

 type and show xylem and phloem elements, together 

 with cambium arcs. There are three types of collat- 

 eral bundles, closed collateral, open collateral and 

 bi-collateral. Closed collateral bundles consist of a 

 xylem portion and a phloem portion separated from 

 each other by a short strip of cambium, the whole 

 being surrounded by a sheath or layer of fibrous tissue. 

 The bundles may be scattered irregularly in the pith 

 and, as a rule, are only present in monocotyledonous 

 rhizomes or stems. Open collateral bundles consist of 

 xylem elements within a cambium zone and phloem 

 elements on the outer side of the cambium. When 

 occurring in roots, this type of bundle may represent 

 secondary development of the primary radial bundles. 



