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1909]  SINNOTT—MESARCH STRUCTURE IN LYCOPODIUM I4I 
leaf-traces, though composed of a comparatively small number of 
cells, show in nearly every case a mesarch structure (jig. 3). 
The remaining species studied have much better developed vascu- 
lar tissues. Their cylinders are in general very similar, being com- 
posed of a much larger number of xylem rays, which end more or 
less acutely. In all the species, metaxylem cells occur just outside 
the protoxylem of the cylinder. In L. éristachyum and L. com- 
planatum var. flabelliforme, this mesarch development is not common, 
but in L. obscurum and L. clavatum, especially the latter, it is very 
noticeable. In vertical section, the centrifugal elements are seen to 
be reticulate, not scalariform, as in the rest of the metaxylem (figs. 
4,6). The exceptional development of this structure in L. clavatum 
may be due to the fact that its vegetative growth is more luxuriant 
than that of any of the other species studied. In all these forms, the 
leaf-trace again is very clearly mesarch (figs. 7-9). ‘This is especially 
conspicuous, perhaps, in L.: tristachyum and L. complanatum vat. 
flabelliforme, where a large protoxylem lacuna occurs in the center of 
the trace (fig. 9). 
In all the species, the leaf-trace, as it leaves the cylinder, is very 
small, but rapidly increases in size in the cortex, where it possibly 
serves to store water. 
The development of the protoxylem of the central cylinder is 
always from without inward. The single row of centrifugal metaxy- 
lem elements occurs directly outside the earliest-formed xylem cells. 
Sections through the growing-point of L. clavatum showed that the 
metaxylem elements on both sides of the protoxylem developed nearly 
simultaneously. In the leaf-trace, also, as was shown by a very 
young stem of L. complanatum var. flabelliforme, the elements sut- 
rounding the protoxylem are all developed at the same time. 
The leaf-trace of Selaginella rupestris (L.) Spring was investigated, 
and though composed of a very small number of cells, it showed 
4 strong tendency toward mesarch arrangement. It is interesting to 
Rote here an observation of Gipson (6, p. 151) on the leaf bundle 
of Selaginella. “In the upper third of the leaf the protoxylem ae 
ments become accompanied by several short reticulate tracheides 
which flank the spiral tracheides, and in section (e. g., of S. Braumit) 
May inclose the spiral elements completely.” 
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