216 SCIENCE PROGRESS. 



the petiole, after repeated branchings gradually lose the 

 phloem from their upper sides, and thus come to possess the 

 collateral structure of the bundle of a Phanerogamic leaf. 

 On the other hand the curved bundle in the petiole of 

 Osmunda is certainly a meristele, if we may judge from its 

 connexion with the bulky central cylinder of the mono- 

 stelic stem, yet it is surrounded by a complete mantle of 

 phloem, and indeed conforms in structure to many true 

 steles (cf. 1 8). We may probably draw the same con- 

 clusion as to the " petiolar steles " of Gleicheniacese (19). 



Similar facts appear to obtain in the polystelic genera of 

 Phanerogams, upon which we may expect much new light 

 from as yet unpublished researches. One instance is, 

 however, too instructive to be omitted. A number of 

 distinct steles arranged in a circle enter the peduncle of 

 Auricula Delavayi (8, p. 304), fuse laterally, and become 

 indistinguishable from a monostele, the central extra-stelar 

 tissue passing over into pith. 1 Van Tieghem warns us (10, 

 p. 768) not to confound such a structure formed in an 

 essentially polystelic stem with an essentially monostelic 

 stem. But if this sort of thing may occur, what guarantee 

 have we that an "essentially monostelic" stem is really 

 essentially monostelic, or, for the matter of that, that an 

 " essentially polystelic " stem is really essentially polystelic ? 

 If a stele can become a collateral bundle in the course of a 

 shoot system, the same transformation may very well occur, 

 or a collateral bundle may become a stele, in the course of 

 descent ; at least we are quite debarred from dogmatically 

 drawing or denying homologies between the one and the 

 other. Of course we can speculate, and in some cases 

 claim a fair degree of probability for our speculations, 

 especially when we have a minute knowledge of all the 

 facts in the anatomy of a given group, but since it is impos- 

 sible to draw a sharp line between a stele and a vascular 

 strand that is not a stele we are clearly not on very firm 

 ground. There is certainly nothing to surprise us in this ; 



1 A similar state of things appears to obtain in some of the Palm roots 

 investigated by Mr. Cormack. 



