102 /Scientific Proceedings, Royal Dublin Society. 



xylem elements are called in the Bracken Fern (though in old 

 rhizomes or underground stems vessels occasionally occur), was 

 filled by well-marked tyloses. It was unfortunately impossible to 

 say what was the cause of their formation, as the section was made 

 from a small, detached piece of the rhizome. 



It would be of interest to see if, in the usual process of cutting 

 bracken, the consequent interference with transpiration in the 

 fern fronds causes a formation of tyloses in the underground stem. 



Tyloses cannot be common in the Bracken Fern, or they would 

 have been observed already, seeing that in the course of class- 

 teaching one sees hundreds of sections without any indication of 

 their presence. 



Though tyloses are, as Molisch and others have shown, found in 

 most of the groups of the flowering plants, according to Kuster, 1 

 the only case on record of tyloses in Pteridophyta, or Vascular 

 Cryptogams, is that of Cyathea insignis, in the old leaf-stalks of 

 which Oonwentz found them. 



There are several points in Kiister's chapter on tyloses which 

 it may not be out of place to mention here. 



Whether found in the cavities of xylem vessels or tracheides, 

 in resin-passages, in secretory sacs, or in the respiratory cavity or 

 air-chamber of stomata, they are the result of Hypertrophy. This 

 term, meaning an abnormal enlargement of a cell, without cell- 

 division, is used in contrast to the term Hyperplasy, In which the 

 abnormality is accompanied by cell-division. Kuster states, con- 

 trary to views now generally held, that a tylose very rarely shows 

 cell-division, and that the diverticulum into the "free space" is 

 not, as usually supposed, cut off by a cell- wall from the parenchy- 

 matous cell giving rise to it. The tyloses which may arise at many 

 points, by the ingrowth, in the case of the xylem, of the surround- 

 ing xylem or conjunctive parenchyma, are due to the local hyper- 

 trophy of that part of the xylem parenchyma opposite the thin part 

 of the wall of the vessel or tracheide, which, through some cause or 

 other needing further investigation, bulges into the cavity of the 

 vessel, and forms a cellular swelling. This, with or without the 

 parent nucleus or a daughter nucleus, combines with similar swell- 

 ings to fill up more or less completely the cavity of the xylem vessel, 



1 " Pathologische Pflanzen-Anatomie," 1903. 



