222 SCIENCE PROGRESS. 



cylinder as such. While we could not admit that the stelar 

 doctrine applied to such stems, we should probably be 

 forced to the conclusion if their vascular system conformed in 

 all other respects to the monostelic type, that the plants in 

 question were derived from truly monostelic ancestors, whose 

 descendants had lost the limit between cortex and cylinder. 



The Nymphseaceae, many of whose stems contain a 

 large number of "scattered" bundles, seem to furnish us 

 with examples of such plants. Caspary (27) states that 

 the bundles are here developed in centripetal order : this 

 would seem to indicate an analogy with those plants 

 (Piperaceae, Begoniacese, etc.), which possess a proper 

 bundle ring" and also younger bundles in the pith, rather 

 than with the monocotyledonous type. In at least one 

 member of the family, Victoria regia, which possesses 

 a particularly large number of these "scattered" bundles, 

 it appears that no well-defined cylinder is visible anywhere 

 in the stem. 1 So here if anywhere we seem to have a real 

 case of "astely". We cannot, however, say the same 

 with certainty of any dicotyledonous stem with a single 

 ring of bundles. Nageli's observations (28) indeed led him 

 to the conclusion that the " cambial " strands were, as 

 a rule, developed in the midst of a homogeneous ground 

 tissue, but his conclusions, as we have seen, have been 

 negatived by most subsequent observers. 



Turning to the vascular cryptogams we find that 

 whether monostelic or polystelic, the stele or steles can be 

 traced nearly up to the stem apex. The first formed peri- 

 clinal walls do not indeed necessarily mark the limit of 

 stelar tissue. They may cut off the pith, as in Equisetum 

 or mark the middle of the cortex, as in many roots, or 

 the outer limit of the ring of steles, as in many fern stems, 

 or of the single cylinder, as in the stolon of Nephrodium 

 (10, pp. 692 and 773-4). Clearly no special importance 

 can be attached to these walls, and we certainly can- 

 not use the fact that they mark off the pith in Equisetum, 



1 1 owe this information to the kindness of a friend in telling me the 

 results of some unpublished observations. 



