1909] 



STOKEY— ANATOMY OF ISOETES m 



not as heavy as in the tracheids of the central axis or of the leaf traces 

 ig. 21). The parenchyma cells, however, have thinner walls and 

 are richer in protoplasm than in the two species just described. Th< 

 parenchyma and the various types of thickened cells are in most cases 

 distributed irregularly, as is shown in fig. 26, although occasionally 

 in the older parts of old stems there are indications of zonation. 



/. Nuttallii shows the same type of cells as /. melanopoda, but there 

 is a difference in arrangement. This species always shows the zona- 

 tion which has been described by Farmer for I. lacustris and by 

 Hegelmaier for /. velata and I. Durum. The parenchyma cells 



cells thick, which alternate with layers com- 

 puacu ui uic various types of thickened cells (fig. 25). The zonation 



evident in voung plants and is very striking in old plants, especially 

 in the older regions of the " prismatic layer." The ^f^^ ™ 



usually entirely empty. r , 



larger, contain more protoplasm, and have thinner walls than those 

 of the first two species. The parenchyma is more like that of L 

 melanopoda 



more 



Nut laid 



than those of the latter species. In the older parts of the stem the 

 thickened empty cells are usuallv collapsed, so that the zones of 

 thick-walled cells, which alternate with the well-developed parenchyma 

 *ones, are apparently much narrower. 



Almost all the writers on Isoetes have called attention to the pies- 

 ace of the fine-grained starch in the cells of the "prismatic layer. 

 This was first noted by Hegelmaier in /. velata and /. Dimaei.m 

 *hich the starch-containing parenchvma cells form zones alternating 

 *ith zones of empty cells. Farmer records the presence of starcn 



» the prismatic layer of /. lacustris, in which the starch-filled ecus 



"e also arranged in zones. The disposition of the starch was touna 

 tovarv^.u • x„ r „,i.:„^^n,-»r FlettiUnd I. melano- 



poda 



van- with the species. In /. echinosp 



matic layer. 



■"^^ uiL 31JCL1CJ5, XLL ± . Clf^/wo^«" * — m u . 



is present abundantly in the cortex but not at all in the p 

 iver » Tn r Tnrh^ani, var. Harveyi, in addition to the 



:a *h in the cortex, there is starch in the parenchyma in the ^vasauar 



axi s, and in tht 



'^one in the << prismatic layer." /. Nuttallii shows what seems to be 

 the more common arrangement in the forms previously descnixa, 

 lhat is, there is abundant starch in the "prismatic layer." 



