140 SCIENCE PROGRESS. 



We may tabulate the results thus obtained as follows: — 



Monostely. — A single central cylinder. All roots and 

 hypocotyls, nearly all Phanerogamic stems, and stems of 

 many Vascular Cryptogams. 



Polystely. — More than one cylinder. Stems of most 

 ferns, most species of Selaginella, and among Phanerogams 

 of Auricula and Gunner a} 



(a) Dialystely.— Steles separate for most of their 

 course. Most Ferns Selaginella and. Auricula ursi, etc. 

 Gunnera. 



(6) Gamostely. — Steles united laterally for most of their 

 course. Marsilia, Pilularia, Pteris aurita, etc. Auri- 

 cula japonica, etc. 



Astely. — No cylinder. Leaf blades, most petioles, 

 stems of some species of Equisetum and Ranunculus^ 

 stems of Hydrocleis, Ophioglossum, Limnanthonum, Nym- 

 phceacece, etc. (yc). 



The publication, in 1890-91, of the second edition of 

 Van Tieghem's Trait e" de Botanique (10), which contains a 

 full exposition of the stelar doctrine on the lines indicated, 

 may be said to mark the close of the second phase in the 

 development of the theory. 



THIRD PHASE— EXTENSIONS AND MODIFICATIONS. 



The third phase, from that date to the present time, has 

 been occupied by various developments and modifications of 

 the doctrine on the part of the author and his pupils, and has 

 been marked by considerable criticism, mainly of these 

 newer developments. 



The first line of research that calls for notice is a re- 

 investigation of the conjunctive tissue of the typical central 

 cylinder of the flowering plant. This has led Flot (11) to 



1 In a paper recently communicated to the Linnean Society, Mr. B. 

 G. Cormack describes cases of polystely met with in the adventitious roots 

 of three genera of Palms, viz., Areca, Cocos and Verschaffeltia. It appears 

 that the single stele of the root splits, as it is traced downwards, into a ring 

 of separate steles. Later on these steles again pass over into a single 

 cylinder. This seems to be an important modification of Polystely as 

 described by Van Tieghem and Douliot, and Leclerc du Sablon. 



