210 PLANT MORPHOLOGY 



young sporoiiliytc lives on the gametophyte until able to maintain itself 

 as an independent plant. 



THE VASCULAR SYSTEM 



The vascular system of pteridophytes and spermatophytes is made up 

 mainly of two kinds of complex tissues: xijlem and 'phloem. Each of these 

 consists of several different kinds of elements. Xylem conducts water 

 and dissolved substances absorbed from the soil, while phloem carries food 

 away from the leaves and other organs where it is synthesized. The unit 

 of tiie xylem is the tracheid—a. slender, elongated, thick-walled cell gen- 

 erally pointed at each end and without living contents when mature. 

 The most important element of the phloem is the sieve tube — an elongated, 

 thin-walled, living cell whose end walls, and often side walls as well, have 

 many fine pores (Fig. 314A, B). 



The cell walls of tracheids are thickened with lignin, which is deposited 

 on the inner surface to form a spiral, rings, parallel bars, or an irregular 

 network, in accordance with which spiral, annular, scalariform, and retic- 

 ulate types are recognized. Most commonly the lignin is so abundant 

 that the walls are pitted, the pits being unthickened areas. Vessels 

 resemble tracheids except that each represents a longitudinal row of cells 

 whose end walls break down. Vessels are of rare occurrence in pterido- 

 phytes but are the chief xylem elements of angiosperms (Fig. 314C--E'). 



In addition to tracheids and/or vessels, xylem may contain paren- 

 chyma. Phloem may contain parenchyma in addition to sieve tubes. 

 Fibers, which are elongated, thick-walled, nonconducting cells, may also 

 form part of the xylem or phloem (Fig. 314F). 



Development of Xylem. A short distance behind the apex of the root 

 and shoot, which is composed of embryonic tissue, the first xylem is differ- 

 entiated. This is known as protoxijlem. The next xylem to lignify is 

 called metaxylem. The position of the metaxylem with reference to the 

 protoxylem is of considerable importance. There are three conditions, 

 as follows: 



(1) If the lignification begins at the outside (periphery) of the root or 

 stem and proceeds toward the center, in a centripetal direction, the devel- 

 opment is exarch. This type is characteristic of all roots and of the stems 

 of lycopods (Figs. 176, 188, 220, 227, and 311). (2) If the lignification 

 spreads out in all directions, so that the metaxylem surrounds the proto- 

 xylem, the development is mesarch. This type is characteristic of the 

 stems of ferns (Fig. 239). (3) If the lignification begins near the center of 

 the stem and proceeds outward, in a centrifugal direction, the develop- 

 ment is endarch. Only a few pteridophytes have reached this condition, 

 but it is almost universal in the stems of spermatophytes. 



Protoxylem consists mainly of spiral and annular tracheids, while meta- 



