350 



CONDUCTING SYSTEM 



spiral. Genuine vessels only occur in exceptional cases (Pteris aquilwa). 

 In the larger bundles parenchymatous cells are interspersed among the 

 tracheides, while other parenchymatous elements separate the latter 

 from the surrounding leptome. The leptome itself consists of sieve- 

 tubes, and of leptome-parenchyma cells, containing abundant protoplasm, 

 which take the place of companion-cells. The entire bundle is enveloped 

 by two concentric sheaths. The inner of these consists of one or more 



Fig. 139. 



One of the smaller bundles (meristeles) from the rhizome of Polypodiwn gJaucn- 

 phyUum in T.S. I, leptome ; ;>, parenchymatous sheath (pericycle) ; e, endodermis ; 

 ?r, the thickened inner walls of the layer immediately external to the endodermis. 

 (The conducting parenchyma is shaded.) x 320. After Potonie". 



layers of starch-containing parenchymatous cells, and may be termed 

 the " phloem-sheath " (Phlocm-scheidc) ; it is surrounded in its turn by 

 a " protective sheath " (Schutzscheide) or endodermis, a structure which 

 will be dealt with in more detail later on. 



The concentric type of vascular structure is foreshadowed, as it were, 

 by the arrangement of the conducting tissue in the [gametophytic] 

 stems of the most highly organised Mosses, namely, the Polytrichaceae. 

 Here the single conducting strand comprises, in the first place, a central 

 water-conducting tissue, which may be regarded as a primitive type of 

 hadrome. In Polytrichuvi juniperinum and P. commune this hadrome 

 is further differentiated into a harder internal region (Fig. 140, t), in 

 which the thickened regions of the cell-wall are red-brown in colour, 

 and a more delicate outer zone with colourless, or at most pale yellow, 

 walls (Fig. 140, ^o); in withered stems this external region of the 

 hadrome is greatly contracted. The central hadrome-strand is sur- 



