420 Scott and Hill.—Structure of Isoetes Hystrix. 
like column of tissue, the cell-rows of which converge upwards, 
towards the central part of the apex. As we follow the 
cylinder downwards from the apex, in successive transverse 
sections, we find that it very rapidly assumes mature structure. 
The leaf-traces, where they abut on the stele, are somewhat 
more differentiated than the stele itself, and have their xylem 
lignified at a level where the central tissue is still in a meri- 
stematic condition. Immediately below this point, however, 
and at a distance not exceeding -15 mm. from the apical surface, 
the differentiation of the stelar wood suddenly begins. The 
lignification at first extends irregularly across the stele, 
connecting the leaf-traces together, but in the next section of 
the series it may be already complete (Fig. 5). Where the 
differentiation is so nearly simultaneous it is perhaps futile to 
seek to determine its direction. In some cases, however, a 
centripetal development can be traced, as shown in Fig. 6, 
where the outer stelar tracheides are fully formed, while the 
central tissue is still undifferentiated. At a slightly lower 
level, the central tracheides are sometimes found partially 
lignified, while those towards the periphery are completely 
formed. 
The primary tracheides are extremely short, corresponding 
to the stunted character of the stem, and their chief diameter 
is usually horizontal. The thickenings on their walls are 
mostly spiral, but annular and reticulate markings also occur 
here and there. 
Among the tracheides a considerable amount of thin-walled 
parenchyma is interspersed, the cells of which retain their 
nuclei, and remain living when the lignified elements between 
them are already disorganized. 
Fig. 5, from the youngest stem at our disposal, which was 
little more than 2 mm. in diameter, gives as far as possible 
a representation of the primary structure of the stele, as seen 
in transverse section. It is however impossible to find the 
structure in a purely primary condition, for the cambial 
divisions begin before the primary differentiation is completed. 
In all the stems examined, whether old or young, tangential 
