THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



115. Transverse section of the protostele of Cheiropleuria, 

 showing a leaf-trace being given off from it. ( x 45. ) 



especially where the protostele is large, as in Lygodiuni, Gleichenia, or Cheiro- 

 pleuria (Fig. 1 15). The xylem is surrounded by a band of phloem, followed 

 by a single or a multiple 

 layer of cells of the peri- 

 cycle, and finally the stele 

 is delimited externally by 

 the continuous sheath of 

 the endodermis. The proto- 

 stele is without intercellular 

 spaces, and the endodermis, 

 being a continuous sheath 

 of cells, serves as an efficient 

 barrier limiting the venti- 

 lating system of the cortex 

 internally, and controlling 

 by its living protoplasts all 

 transit of materials into or 

 out of the stele. The par- 

 enchymatous cells within 

 this sheath are sometimes Fig. 

 spoken of collectively as 

 conjunctive parenchyma. They permeate the phloem, and often also the 

 xylem-core itself Thus constructed the protostele receives the leaf-trace 

 from each successive leaf which, passing through the cortex, inserts itself 

 upon and fuses with the stele of the stem. The several tissues of the one 

 come into intimate continuity with those of the other. It is important to 

 note that the leaf-trace in typically protostelic Ferns is itself surrounded by 

 endodermis, and its entry is effected without any break of continuity of the 

 enveloping endodermis, and without any marked disturbance of the core of 

 xylem. The root-traces also connect directly with the protostele ; but they 

 arise in less definite order and cause even less disturbance of the stelar 

 structure than the leaf-traces do. Thus constructed the simple vascular 

 system serves the whole plant, and for small plants such as young sporelings 

 the non-medullated protostele appears to be suitable and efficient. 



In certain Ferns the protostelic state is maintained throughout the life 

 of the adult. This is so in Botryopteris cylindrica (Fig. 1 14) and other ancient 

 fossils. It is also found in the living genera Gleichenia, Lygodium, and 

 Cheiropleuria, as well as in the whole family of the Hymenophyllaceae 

 (Fig. 116). This is, however, uncommon for the adult stems of Ferns. 

 As a rule in them the stele expands into various complicated forms. Some 

 explanation of the permanent retention of the protostele in such cases as 

 those named may be found in the fact that, for instance, in Gleichenia and 



