736 MINNESOTA BOTANICAL STUDIES. 



parable with that condition so fully studied in the stems of 

 Coniferas 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 Pterygophora 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 lacunae 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 



