468 SCIENCE PROGRESS. 



divides up, above the hypocotyl, so that the transverse 

 section shows a number of equivalent steles, each of which 

 repeats in essentials the characters of the single central 

 cylinder of the embryonic stem. This is the case (according 

 to the interpretation of Van Tieghem and his school) in most 

 Ferns, and in many Selaginellas, as well as in the excep- 

 tional Dicotyledons above mentioned (9 and 5). Accord- 

 ing to De Bary's terminology each stele of these polystelic 

 stems is simply a single concentric bundle. The researches 

 of Leclerc du Sablon (10), who has investigated the transi- 

 tion from root to stem in many Ferns, seem to show con- 

 clusively that here, at any rate, the strands in question are 

 best regarded as steles. We must not, however, exagger- 

 ate the importance of the distinction. Every gradation can 

 be found between the most obvious stele and a single 

 vascular bundle. The transition can, for example, be traced 

 in following the vascular system of the leaf in almost any 

 Fern. We reach a point at which the structure of the stele 

 is so simplified that it becomes a matter of indifference 

 whether the term stele or bundle be used. Even in the 

 stems and petioles of Marattiacese, where the steles are of 

 large size, and must each be regarded as including a number 

 of vascular bundles, we find a structure very different from 

 that of the typical central cylinder of a monostelic stem ; for 

 all the protoxylem groups are on the same side of the wood, 

 being directed towards the centre of the organ. Here, and 

 in many similar cases, the strand termed by Van Tieghem 

 a stele, while representing much more than a single vascu- 

 lar bundle, does not really correspond in structure with the 

 entire cylinder of an ordinary stein, though in other cases 

 again, the correspondence is complete. Still, the fact re- 

 mains, that in the plants in question the central cylinder ot 

 the embryo is not continued as such into the upper part of 

 the stem, but divides up, or branches out, into a number of 

 distinct stelic masses, which lie embedded in the paren- 

 chyma. 



Polystely, though so remarkable a modification from 

 an anatomical point of view, is not necessarily of any im- 

 portance as a taxonomic character, for not only do closely 



