320 BOTANICAL GAZETTE [apri 



melanopoda, I. Tuckermani var. Harveyi, I. echinospora var. Flettii, I. 

 Nuttallii. A series according to the size of the sporangia is a- follow^ 

 /. Nuttallii , /. melanopoda, I. echinospora var. Flettii, I. Tucker hianl 

 var. Harveyi. It is perhaps not safe to generalize from a comparison 

 of only four species, but it is worthy of- note that the size and 

 development of the bundle in these forms is not related to the 

 size of leaf, but follows the other two series, that of habitat and six 

 of sporangium. 



Hill (13) calls attention to the presence of two canals in the leave- 

 of I. Hystrix, which he regards as representing the parichnos of the 

 Lepidodendreae. In none of the four species examined is ther 

 any trace of a canal or any indication of a tendency to form canak 

 In this respect these species agree with /. lacustris. 



THE STEM 



is well known, the stem of Isoetes is a short tuberous bod} 

 whose vascular axis is very small in proportion to the diameter of the- 



stem. 



very 



left in a deep pit by the overgrowth of the surrounding region. If 

 the term stele may be applied to a region whose connection with a 

 plerome is far from certain, then the vascular axis may be denned I 

 a non-medullated monostele, consisting of xylem and parenchyma 

 forming a loose network. The xylem is made up of short spiral, 

 annular, and netted tracheids, whose long axis is transverse to the 

 stem (fig. 22). The parenchyma cells contain abundant protoplasm, 

 and the nuclei appear active. The xylem axis is surrounded by 

 similar parenchyma cells, one to three layers deep, but in neit er 

 young nor old plants is there a trace of phloem. A great many youn 

 plants of I. echinospora and a few of /. Tuckermani var. Harvev 

 were examined, and in all cases the xylem is surrounded by undi er- 

 entiated parenchyma. In very young plants the vascular axi> »> 

 exceedingly small, but there is a constant increase both in length an 

 diameter with the increase in the number of leaves. . 



The differentiation into protoxylem and metaxylem. *[?. 

 usually well marked in pteridophytes, is lacking in Isoetes. T e» 

 neither a difference in the time of development nor in the chaw ^ 

 of the elements. A difference in character of elements is not like ) 



