RACHIOPTERIS CYLINDRICA. 61 



scalariform structures quite similar to the larger ones, save for the 

 smaller diameter. The same may be said of the oblique transverse 

 sections, where the markings of the large and small elements appear to 

 be the same. In No. 127, however, there are suggestions at one or two 

 points of a spiral marking, but whether these are actually spiral, or are 

 really scalariform markings altered in some way, it is impossible to say. 



The position then as regards the xylem is this. In tranverse sec- 

 tions we have one or more groups of small elements that may 

 be interpreted as groups of protoxylem, but this interpretation has not 

 so far been confirmed by satisfactory evidence that they have spiral or 

 annular markings. 



Neither in transverse nor longitudinal sections have any cellular 

 elements been met with in the xylem, so that the xylem may be said 

 to be wholly vascular, so far as observation has at present gone. 



Division of the Stele. 



The description of the stele given above does not apply to all the 

 preparations that have been examined. In the majority a condition is 

 met with which does not appear to have presented itself to Williamson 

 at the time he dealt with this plant, and which shows the stele in a 

 state of division (Figs. 2, 3, 4, 5). As this appears to be associated 

 with the mode of branching of the plant, and the formation of lateral 

 members, it deserves to be described with some detail. 



So far as has yet been seen, the division of the stele takes place in 

 one of two ways, being either (1) an equal division, or (2) an unequal 

 division. 



In equal division, the stele divides along a diameter in such a way 

 that the two halves have, from the first, the same size, form, and ap- 

 pearance, and when the process is completed we get two distinct steles 

 of the type already described. An early stage of the process is well 

 shown in No. 110, where we have two semi-circular masses of xylem, 

 inclosed in a common zone of phloem, and separated by a narrow band 

 of thin-walled parenchyma. This parenchyma cuts through the xylem 

 in such a way as to have the small groups of elements, presumed to be 

 protoxylem, abutting directly upon it. As the two moieties of the 

 stele become more and more divergent, a centrifugal development of 

 xylem would seem to take place on the inner side of these semi-circular 



