242 PLANT MORPHOLOGY 



tAvo-lobed or three-lohed, depending on the species. It is covered by over- 

 lapping leaf bases. Between its lobes arise numerous dichotomously 

 brancired roots. As in Selaginella, each leaf has a ligule, arising at its 

 base on the adaxial side. The root and stem grow by means of a meri- 

 stem (Fig. 199). The stem has both an apical and a basal meristem. 



Vascular Anatomy. Each leaf has a single vascular bundle and four 

 long air passages with numerous transverse partitions. Stomata are pres- 

 ent only on leaves exposed to the air. The stem structure is rather com- 

 phcated and difficult to interpret. Many botanists regard the upper part 



Fig. 199. Longitudinal section through the stem tip of Isoetes howellii, showing the apica 

 meristem and developing sporophylls, each with a prominent ligule, X300. 



of the stem, which bears the leaves, as the stem proper and the lower por- 

 tion, which bears the roots, as a rhizophore, although no such differentia- 

 tion is evident externally. 



The vascular cylinder, representing a greatly reduced protostele, is 

 surrounded by an extensive cortex (Fig. 200). A notable feature of the 

 stem of Isoetes is the occurrence of secondary thickening. The primary 

 xylem, consisting of extremely short tracheids intermixed with consider- 

 able parenchyma, is surrounded by a narrow zone of primary phloem. 

 This, in turn, is enclosed by a "prismatic layer," which represents the 

 internal product of cambial activity. On the outside the cambium adds 

 new tissue to the cortex. This tissue, which is parenchymatous, has the 

 position of secondary phloem but not its structure. The tissues forming 

 the prismatic layer are not uniform, but are differentiated into alternating 

 zones of thin-walled and thick-walled cells. The thin-Avalled cells are 

 ordinary parenchyma, while the thick-walled cells are lignified and have 

 scalariform and reticulate markings. Thus the prismatic layer has the 

 position of secondary xylem but not its typical structure. Whether the 



