736 MINNESOTA BOTANICAL STUDIES. 
parable with that condition so fully studied in the stems of 
Coniferze and woody Dicotyledons. In other instances, how- 
ever, the rings in the stipe seem to arise quite as in the 
hapteres. 
There is in the stipe of Plerygophora a plain distinction be- 
tween the first or primary structure of the cortex and the second- 
ary structure which is established in the lacuniferous period of 
growth. It is at this time, during the first year, that the outer 
cortex readily peels from the inner. Later, with the resump- 
tion of growth in thickness, the lacune are filled by the radially 
dividing cambial tissue. The tissue of the first or innermost 
ring, surrounding the pith, differs in appearance from that of 
subsequent rings. The cells diminish in diameter towards the 
periphery of the ring until they have the look of stereome in 
cross section. In this part of the ring the cells are very strongly 
pitted, in marked contrast to the inner cells of the primary cor- 
tex where the pits are but occasional. All the cells, however, 
of the secondary cortex as displayed in subsequent growth- 
rings are strongly pitted. Another distinction between the cells 
of the innermost ring and those of subsequent rings is that the 
elements of the first ring are slenderer, more prosenchymatous, 
almost approaching the fibrous shape peripherally, while the 
elements of later rings are shorter, more parenchymatous and 
not at all suggestive of fibrous tissue. 
In older stipes the pith web is decidedly solid, in marked con- 
tradistinction to this tissue in young material. As the stipe 
matures the interstices between the elements of the pith web 
become obliterated by the repeated branching and interlacing of 
the filaments. The chlorophyll also disappears and the cells 
become filled with densely granular contents. There remain, 
however, in even the oldest pith, numerous interstitial passages 
which, in cross section or longitudinal section, present much the 
same appearance and are, perhaps, what were mistaken for 
mucilage ducts by Ruprecht. 
A cross section of the mature stipe shows then the following 
characters. At the center is the solidly interwoven tissue of 
the pith web. This is surrounded by the clearly marked, 
sclerenchymatous tissue of the primary cortex passing insensi- 
bly into the tracheid-like tissue of the secondary cortex which 
is arranged in concentric rings, resulting from the succession 
of elements with larger cell-lumina in apposition upon those 
