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SCIENCE PROGRESS 



the monostele ; whether the gamodesmic system is arrived at 

 by the reunion of separate meristeles ; and so on. 



2. It will be observed that the endodermis plays an important 

 part, inasmuch as it is looked upon as the delimiting layer 

 between the stele and the cortex. The question which arises is 

 whether this endodermis has the morphological value attributed 

 to it ; if it has, then the theory may be justified. 



Since the Ferns provide us with a greater diversity of 

 vascular structure than the Phanerogams, it may be considered 

 desirable first to follow the development in any convenient 

 plant belonging to the former class. 



A young plant of Doodia aspera R. Br. (see Chandler, 10) has 

 a central cylinder consisting of a solid mass of vascular tissue, 

 a rod of xylem or wood surrounded by phloem or bast, and 





Eldoderm/s 



Leaf-Pace 



Solenostele. Diagram of aTrnnsverse- 

 action tfvrougK ar\ internode-. 



'Leaf-gab 



Sole/\OS"tele. Diagram oi afraWerSt- 

 sedTon mrooaK anode. 



the whole encircled by an endodermis. This type of structure 

 has been termed the protostele. As development proceeds, a 

 small collection of phloem elements appears in the centre of the 

 xylem. The first leaf-trace bundle— concentric in nature — is 

 formed by the outward passage of a portion of the vascular 

 tissue ; this leaves a gap in the central cylinder so that a 

 transverse section through the node shows a horseshoe-shaped 

 mass of xylem, and the internal and external phloem in 

 continuity (fig. 4, a). 



After the exit of the foliar strand, the leaf-gap is closed by the 

 horns of the wood gradually coming together; hence there results 

 a cylinder of xylem bounded both internally and externally by 

 phloem, and the whole enclosed within the endodermis (fig. 4, b). 



As the formation of successive leaf-traces is followed, it is 

 found that the gap formed in the central cylinder by the outgoing 



