6o8 



FILICALES 



From this point, therefore, up to the third or fourth leaf, the centre of 

 the xylem-strand is occupied by a core of phloem. At the departure of 

 about the third or fourth leaf the pericycle follows the phloem down into 

 the internode below, so that a few pericyclic cells are now to be found in 

 the centre of the core of phloem. At the fifth leaf 

 (or sometimes at the fourth) the endodermis also 

 is decurrent, giving rise at first to a few cells only 

 in the centre of the pericycle, which usually dis- 

 appear before the node below is reached. Higher 

 up it is continuous from node to node, and sur- 

 rounds a progressively increasing amount of ground- 

 tissue, which is now decurrent with it. The vascular 

 system has, in fact, become a solenostele. This 

 stage, however, does not last long, for the leaf-gaps 

 begin to overlap after the departure of about the 

 eighth leaf, and above this point the system 

 becomes more and more dictyostelic, although at 

 first a complete vascular ring is occasionally to be 

 met with. The leaf-trace of the first five or six 

 leaves consists of a single curved strand. Above 

 this point two or three separate strands are given 

 off to each leaf, and at about the tenth leaf four 

 such strands are present, two arising from each 

 side of the leaf-gap. 



The first indication of the internal steles that 

 occur in the mature plant is to be found at about 

 the tenth leaf. Just below one or both of the 

 two upper (adaxial) traces of this leaf the xylem 

 of the stem-stele is seen to project slightly inwards, 

 so as to form a small ridge on its internal surface, 

 which is often continued as such for some distance 

 down the stem. Sometimes, however, it separates 

 off completely so as to produce a small xylem- 

 strand lying free within the phloem of the stele, 

 which either ends blindly below or eventually fuses 

 up again with the main xylem-strand. These free 

 xylem-strands are always present at the subsequent 

 leaf-gaps, and, although still remaining enclosed by 

 the same endodermis, they become more and more distinct from the main 

 xylem-strand of the stele. Later on they may even separate off from the 

 stele altogether in the upper part of their course, only fusing with it 

 again at a point lower down. The separation of the small xylem-strands 

 from the main stele finally becomes complete throughout, and from 

 their starting-point they run as small independent vascular strands ending 

 blindly in the central ground-tissue, and having no further communication 



FIG. 338. 



Alsophila excelsa. Diagram 

 of vascular system of a young 

 plant in median longitudinal 

 section. The xylem is black, 

 the phloem lightly shaded, and 

 the endodermis is indicated by 

 a dotted line, the ground-tissue 

 is left white. (After Gwynne- 

 Vaughan.) 



