1922] 



PFEIFFER MONOGRAPH OF THE ISOETACEAE 85 



rows, somewhat obscuring 1 the real form of the stem in many 

 cases. 



The upper portion of the stem or corm is flattish or concave; 

 in growth, the central lowest part gives rise to the newest leaves, 

 while the successively older ones are pushed out to the periphery 

 by the newer growth. There is no apical cell responsible for 

 stem growth, according to Farmer ('90), although according to 

 the evidence seen by Scott and Hill ('00) there may be one. 



Anatomically, the compact stem has proved an interesting 

 problem. The vascular cells are centrally located in a single 

 group, which has been interpreted either merely as a union of 

 the leaf -traces (Hegelmaier, '72) or as a small distinct stem stele 

 plus the leaf-traces (Scott and Hill, '00). 



Although the xylem character of the tracheids in this central 

 region seems clear, there has been much discussion as to the 

 significance of the zone immediately about it, the so-called 

 "prismatic layer." According to Russow ('72) and Scott and 

 Hill ('00) this undoubtedly contains distinct phloem elements. 

 According to Farmer ('00), Wilson-Smith ('00), and Stokey 

 ('09), the phloem interpretation is invalid, because of the in- 

 ability to identify sieve structures here. The last-named worker 

 sees only xylem cells as representatives of conducting tissues 

 in this region. 



A meristematic zone add^ new tissue to the prismatic layer 

 and to the outer parenchyma region. The latter, cortex in nature 

 and position, usually dies at the margin, and in time is sloughed 

 off, although the accumulation of dead cells may be appreciable 

 before being worn off. This is especially true in terrestrial forms 

 where the leaf bases are persistent and the dead cortex remains 

 in place over successive seasons. More frequently, however, the 

 wearing away tends to balance in part the increase in diameter, 

 so that even very old stems are not excessive in size. 



The roots are interesting in that they branch dichotomously. 

 In anatomy, they are peculiar in being collateral endarch. The 

 vascular bundle upon emergence from the central stele is sur- 

 rounded by parenchyma which is replaced by a small group of 

 phloem cells on the side away from the axis of the stem (Stokey, 

 '09). 



In the early growth of the sporophyte and before sporophyll 

 production in each growing season, sterile leaves are usually 

 produced. These are similar to the fertile leaves or sporophylls 



