STEMS 



679 



absent or poorly developed in submersed aquatics, where the food- 

 making organs are likewise organs of water absorption. But in all 

 large land plants, where the organs of synthesis 

 are remote from the organs of absorption, con- 

 ductive tissues are well developed. The sub- 

 stances that migrate from cell to cell are either 

 raw materials (water and inorganic salts) or 

 organic foods commonly manufactured by the 

 plants themselves, water being quantitatively 

 much the most important migrating substance. 

 These materials move from regions of higher 

 to regions of lower pressure, water and soil salts 

 ascending from the roots to the leaves, and 

 organic foods moving in various directions from 

 the seat of manufacture in the leaf. 



The structure of the conductive elements. — 

 Tracheids and tracheae. — When conductive 

 cells begin to differentiate from other cells, 

 their chief distinctive feature is elongation, 

 which remains the most fundamental char- 

 acteristic common to all conductive elements. 

 The commonest conductive elements are tracheae 

 (also called vessels or ducts) and tracheids, which 

 together are sometimes called hydroids, a term 

 suggestive of their role in water conduction. 

 Tracheids arise through the differentiation of 

 certain parenchyma cells which elongate and 

 enlarge, the walls also becoming lignified {i.e. 

 woody); at maturity they commonly are pro- 

 senchymatous, that is, their ends are pointed, 

 owing to the development of oblique walls from 

 originally transverse terminal walls (figs. 907, 

 936). Tracheae differ from tracheids in being 

 cell fusions or syncytes, arising through the re- 

 sorption of the end walls. Thus a tracheid may 

 represent only a stage in the development of a 

 trachea, as in young angiosperm tissue, all 

 transitions sometimes being observed (figs. 1000- 

 1002). However, in ferns and conifers and in 



Figs, iooo, iooi. — A 

 portion of a scalariform 

 vessel (trachea) from the 

 root of the prickly lettuce 

 (Lactuca scanola): looo, 

 a longitudinal view, show- 

 ing how tracheae arise 

 from tracheids through the 

 resorption of the cross 

 walls; note that these 

 walls have been resorbed 

 except for rings of tissue 

 (r) next to the longitudinal 

 walls ; note also the trans- 

 versely elongated pits (p), 

 characteristic of scalari- 

 form vessels ; loo r , a cross 

 section of such a scalari- 

 form vessel; both figures 

 highly magnified. 



