PTERIDOPHYTA 141 



round the nodes or joints of the stem. These reduced 

 leaves are practically useless as assimilative organs, and 

 their office is assumed by the internodes of the stem 

 and branches, where the green tissue is largely devel- 

 oped, and connected with the outside by numerous 

 stomata in the epidermis. The leaves serve as protec- 

 tive organs only, forming a thick covering over the 

 apex of the young shoot, and also covering the buds 

 from which spring the lateral branches. 



In studying the development of the tissues of the 

 sporophyte, one is struck by the almost mathematical 

 regularity in the divisions of the cells at the stem-apex, 

 as well as in the roots. The shoot in all species termi- 

 nates in a single apical cell (Fig. 36, C), having the 

 form of an inverted three-sided pyramid from whose 

 lateral faces segments are cut off in regular succession, 

 and the tissues of the mature stem bear a definite 

 relation to the early divisions in these segments. A 

 similar regularity exists in the early divisions of the 

 cells at the apex of the root. The stem is traversed by 

 a regular system of lacunae, or air-passages (Fig. 36, 

 B, ), and the vascular bundles are arranged in a circle, 

 recalling the arrangement in the stem of the typical 

 Dicotyledons. In the arrangement of the woody tissue 

 and bast, they recall the flowering plants rather than 

 the ferns, although among the latter the Ophioglossacese 

 show a somewhat similar type of vascular bundle, the 

 "collateral" form, and also other structural resem- 

 blances. 



The sporangia of Equisetum occur upon peculiar um- 

 brella-shaped sporophylls which are arranged in whorls 

 about the apex of certain shoots, and crowded together so 



