236 



GENERAL BIOLOGY 



As already stated, each group of vessels with its connecting mechan- 

 ical or supporting tissue forms a fibre-vascular bundle. These may be 



either complete or incomplete; com- 

 plete, if they possess xylem, phloem, 

 and cambium elements, and incom- 

 plete if they possess either xylem or 

 phloem without the cambium ele- 

 ment. The xylem is always sup- 

 ported by wood fibers and the 

 phloem by bast fibers. There are 

 five different arrangements of fibro- 

 vascular bundles : 



(1) Radial (common in all 

 young roots, and sometimes in 

 mature monocotyledonous roots). 

 These are always incomplete, con- 

 sisting of either xylem or phloem. 

 The xylem or phloem elements are 

 arranged in a circle within the endo- 

 dermis, a xylem bundle alternating 

 with a phloem bundle. 



(2) Concentric fibre-vascular 

 bundles (common in monocotyledo- 

 nous roots and stems), are bundles 

 consisting of both xylem and 

 phloem, so arranged that either 

 the xylem surrounds the phloem or 



the phloem surrounds the xylem. The former arrangement is the more 

 common. The bundles are irregularly scattered in the pith region. 



(3, 4, and 5) Collateral fibre-vascular bundles are complete, having 

 both xylem and phloem elements, as well as a cambium-arc. These are 

 in turn divided into three types, known as open, closed, and bi-collateral. 



(3) Closed Collateral bundles (usually found only in the pith of 

 monocotyledonous stems and rhizomes and the leaves of all seed plants), 

 are made up of a xylem portion and a phloem portion, never separated 

 from each other by a strip of cambium. 



(4) Open Collateral bundles (most frequently found in dicotyledo- 

 nous roots and stems) are made up of xylem elements within a cambium 

 zone and phloem elements on the outer side of the cambium. 



(5) Bi-Collateral bundles (found in some dicotyledonous roots and 

 stems) are made up of a xylem element and the associated cambium, but 

 with two phloem elements, one on each surface of the xylem. 



Assimilating and Synthesis Tissues (Fig. 140). The Chloroplasts 



Fig. 138. Absorption Tissues. 

 1. Root hairs (H) on rootlet of germinat- 

 ing Fenugrek seed. C, root cap. 2. Stomata, 

 surface view. A, breathing pore. G, guard 

 cells. B, bordering, neighboring or surround- 

 ing cells. 3. Stomata, sectional view. A, 

 breathing pore. G, guard cells. B, bordering 

 cells. 4. Lenticel (A). (From C. W. Bal- 

 lard's "Vegetable Histology," Courtesy of 

 John Wiley & Sons). 



