CONDUCTING TISSUES 127 



structures extending through the parenchymatic tis- 

 sues. The contents of latex tubes may become brown 

 in the preparation of specimens and this coloration 

 aids in tracing the course of the vessel. The walls 

 are very thin and are composed of cellulose. 



Porous Parenchyma. — The parenchymatic tissues 

 of the plant may aid in the distribution of nutrients 

 by reason of the fact that nutrients in solution readily 

 dialyze through their thin cellulose walls. Parenchyma 

 cell walls, especially those in the pith or central region 

 of the plant are usually perforated (Plate 46, No. 4) ; 

 and the transfer of liquids through these cells is 

 fairly rapid. 



FrBRO-VASCULAR TISSUES 



As the tissues concerned in the transportation of 

 nutrients are usually delicate structures, they are sup- 

 ported by fibrous elements. Each group of vessels, 

 with the adjacent mechanical or supporting tissue, 

 is termed a fibro- vascular bundle. As previously 

 noted in Chapter VI, the fibro- vascular bundles may 

 be complete or incomplete. The complete bundles 

 consist of xylem, phloem and cambium elements, 

 whereas the incomplete bundles consist of either 

 xylem or phloem tissues, cambium arcs being absent. 

 The xylem bundles consist of a vascular element, ducts 

 or vessels, supported by a fibrous element, xylem or 

 wood fibers. The phloem bundles likewise consist 

 of a vascular element, sieve tubes, supported by a 

 fibrous element, bast fibers. 



The tissues composing a fibro-vascular bundle 

 may be arranged in any one of several different ways. 

 Because of these differences in arrangement, five types 



