324 BOTANICAL GAZETTE [MAY 
present in all of them. Sometimes this central stele was observed 
in every other internode, but I found also some cases where the apical 
and basal internodes possessed such a central stele, but not the two 
internodes between them. 
In the basal internodes of the aerial shoots the structure of the 
stele becomes more regularly developed, the leptome and hadrome 
forming an equally broad ring around the central pith, which conse- 
quently becomes much reduced in width; in other words, these steles 
show exactly the same concentric and uniform structure as the central 
stele of the monostelic axes. 
In very small vegetative shoots, developed in the axils of the leaves 
of the larger floral ones, the structure is as described above, but the 
number of steles may vary. For instance, in a shoot consisting only 
of two short internodes, the basal one contained only four steles, all 
peripheral, while the upper one had one central, two large peripheral, 
and two still broader ones, each of which consisted of two that had 
partly fused together; the latter corresponding in position with the 
four marked S in fig. 9. In these internodes the structure and dispo- 
sition of the collenchyma and cortex was exactly as described above. 
In order to ascertain the number of steles in the stem of Dianthera, 
I examined shoots of very different height and thickness, but the 
number never exceeded seven—six peripheral and one central. A 
very thick branch with two opposite and equally robust shoots showed 
seven steles above and below the node, and also in the two lateral 
shoots. It is somewhat surprising to see that this polystelic structure 
ceases at once when the axillary shoot is an inflorescence and not a 
vegetative branch. The short inflorescence of Dianthera americana 
is borne upon a long, naked internode, and this internode is mono- 
stelic from base to apex. In this the cortex incloses completely one 
broad central cylinder with one zone of leptome and one of hadrome, 
with parenchymatic rays of equal size and with a central cylindric 
pith, as is characteristic of monostelic axes. 
In the genus Primula the Danish botanist VAUPELL” detected the 
polystelic structure in P. Auricula, and VAN TreGHEM and DoULIOT 
2 Om Rhizomets Bygning hos Primula auricula og chinensis. Vidensk. Medd. 
iecasibe For. Kjébenhaven 1849:76; and Untersuchungen iiber das peripherische 
Wachsthum der Gefassbiindel der dicotyledonen Rhizome. Leipzig. 1855. 
