THE STELA R THEORY. 139 



taken by De Bary in his classical Vergleichende Anatotnie. 

 The numerous vascular strands in the rhizomes of most 

 Leptosporangiate Ferns were regarded by De Bary in the 

 same light. 



But Van Tieghem, having, as we have seen, come 

 to regard the central cylinder rather than the bundle as 

 the morphological unit of vascular tissue in both root and 

 shoot, was now led to the conclusion that in Auricula, 

 Gunnera and the majority of Ferns 1 we have really to 

 deal with a splitting- of the single cylinder of the hypocotyl, 

 as we trace it upwards, by successive bifurcations, into a 

 number of such cylinders (jc and 8). Van Tieghem 

 and Douliot proposed to call such a cylinder a stele (Greek 

 <7r/;X»7, a column). A root or a stem containing one such 

 stele would be monostelic, if it contained more than one 

 polystelic. A third case was distinguished. If the cylinder 

 of the hypocotyl breaks up, as it is traced upwards, into 

 its component bundles, each of which is surrounded by a 

 special endodermis, the cylinder, according to our authors, 

 no longer exists ; the stem is astelic. This case, already 

 described in 1872, obtains in the stems of various Ranun- 

 culacece, in Nymphceacece, in Hydrocleis, in some species of 

 Equisetum, etc., as well as in the majority of petioles and in 

 blades of all leaves. 



Cases of Polystely fall into two groups. First, where 

 on a transverse section the various steles are seen to be 

 completely separate, we have a state of dialystely. Secondly, 

 where the steles are united laterally, so as to form a more or 

 less complete ring in transverse section, enclosing a more or 

 less isolated portion of extra-stelar tissue, which occupies the 

 centre of the ring, we have a state of gamostely. These 

 two conditions are not to be sharply separated, since the 

 steles of all polystelic stems show more or less frequent 

 lateral unions, and the gamostelic condition is simply a 

 case where these unions are very frequent and persistent. 



1 Leclerc du Sablon in 1890 (9) worked out the connections, in 

 several Ferns, of the single hypocotyledonary cylinder with the cylinders 

 of stem. 



