COXDUCTIXG SYSTEM. 



l^f) 



Fig. 52.— Tracheids of Pine wood radial lomc 

 section with border pores, m. medullary rays. 

 (Tscliirch). 



3. Parenchyma sheath, conducting 

 soluble plastic materials. 



In this classification it will be noticed 

 that the bundle is considered apart from 

 its mechanical supports, and that the 

 terms Hadroni and Ijeptom are synony- 

 mous with Xylem and Phloem, from 

 which the wood fibres and bast fibres 

 have been omitted. 



Much variation will be found in indi- 

 vidual bundles, iu the number and kinds 

 of elements present, some bundles con- 

 taining only one or two of the elements. 

 Such variations should be borne in mind 

 in the practical study of tissues. In gen- 

 eral, however, anatomists recognize as 

 component parts of the conducting sys- 

 tem the foilowing: The vessels, tra- 

 cheids, and conducting parenchyma in 

 the Hadrom; the Sieve tubes, Accom- 

 panying cells (Geleitzellen), and Paren- 

 chym in the Leptom, 



1. The Vessels form the most con- 

 spicuous iK)rtion of the internal struct- 

 ure of the plant. Their function is to 



carry water with inor^'anic salts in solu- 

 tion up from the root hairs to mingle it 

 with llie i>histic materials that are to he 

 brought down from the leaves. The v» > 

 sela are of two main types. Ducts and 



Fiff. 52. a.— Tracheids of Pine wood on c: 

 section, m. medullary rays; c, fall wood, *. resin 

 passage In Bprine wood—border pores seen in 

 cross section b. (R. Ilartig). 



Tracheids, both being by some authors 

 included under the name of Trachea. 



The Tracheids are of a more elemen- 

 tary type of cell and make their appear- 

 ance lower down in the vegetable world 

 than do the Ducts. In the lower plants 

 they are simply elongated cells with a 

 well-marked lumen and oblique cross 

 walls. They first assume their charac- 



Fig. 53.— Tyi>es of ducts, r. annular, s. #., #i- 

 spiral ducts, with simple and double spirals, « 

 reticulated. 



