Anatomie. 1 17 



dichotomy of one of thc primary forks. This view of the leaf 

 gives Support to a Gleiclienlaceous affinity for Matonia. 



The successive complications passed through by the vas- 

 cular System in the young plants may be roughly summarized 

 as follows: At its simplest the vascular System consists of a 

 slender central solid protostele without any phloem at all. The 

 departure of the first leaf-trace does not in any way aifect its 

 structure. Just above it a core phloem appears in the centre 

 of the xylem whicli is continuous with the phloem that has 

 appeared on the outside of the xylem through a gap in the 

 xylem ring formed by the departure of the second leaf-trace 

 (Lindsaya-iy^e). The second leaf-trace does not Interrupt the 

 Stele as a whole. Above this point endodermis cells appear 

 in the phloem core which become continuous with the external 

 endodermis at the third leaf-gap. The xylem at the margin of 

 the third leaf-gap projects inwards into the internal phloem so 

 as to form a ridge which in the upper internodes eventually 

 becomes a free xylem Strand. In the neighbourhood of each 

 node the free xylem Strand enters into connection with the 

 external xylem ring in the mid-dorsal line. As the stele in- 

 creases in size a pith appears within the internal endodermis. 

 The pith is not at first connected with the external ground- 

 tissue at the nodes although later on it becomes so. In the 

 meanwhile the internal xylem Strand has obtained a phloem 

 ring of its own and it then becomes completely separated off 

 from the vascular ring throughout the internode by means of 

 the internal endodermis. In a more advanced condition the 

 phloem of the internal Strand extends from one side into the 

 middle of the xylem; this is followed by the endodermis and 

 finally by the internal ground-tissue, so that the internal Strand 

 takes the form of a hollow cylinder. These changes first 

 appear at the nodes from where they gradually extend throug- 

 hout the internodes. At the earlier nodes this second cylinder 

 sends up a distinct column of tissue to fuse with the normal 

 solenostele so as to close the leaf-gaps. Later on the column 

 of tissue also contributes a Strand of tracheides to the adapical 

 free ends of the leaf-trace In the largest and most advanced 

 nodes the connection with the solenostele is made by the roof 

 itself of the second cylinder. It is gradually raised up as a 

 broad flattened plate which both closes the leaf-gap and forms 

 the main vascular supply of the incurved adaxial limbs of the 

 leaf-trace. Contemporaneously with these changes a third 

 cylinder, eventually hollow, appears in the central ground-tissue 

 within the second cylinder. The third cylinder is attached to 

 the second at each node at or near the point of closure of 

 the gap in the latter. 



The authors have made it clear that the two internal 

 hollow cylinders are developed from an originally solid vascular 

 Strand in a manner essentially similar to that in which the 

 outer solenostele arises from the protostele of the young plant. 



