PTERIDOPHYTA. 229 



426. The epidermal cells are mostly narrow and elon- 

 gated. The breathing-pores, which are present in all the 

 chlorophyll-bearing parts of the plant, are arranged with 

 more or less regularity in longitudinal rows; on the stem 

 they occur in the channels between the numerous ridges. 

 The fibro-vascular bundles of the stem are disposed in 

 a circle, and run parallel with each other from node to 

 node, where they join with one another. They contain 

 tracheary, sieve, and fibrous tissues, arranged somewhat 

 as they are in the bundles of flowering plants. 



427. The spores of Horsetails are produced in cones at 

 the summit of the stems. The cones are composed of 

 crowded whorls of shield-shaped leaves, each of which 

 bears upon its under surface fi.ve to ten spore-cases (Fig. 

 137, B). The spores are spherical, and at maturity 

 the outer wall splits spirally into four narrow filaments 

 (elaters) which unroll when dry, and roll up around the 

 spore again when moistened. Their office seems to be to 

 aid in setting the spores free from the spore-cases. The 

 spores germinate soon after falling upon water or moist earth, 

 enlarging and successively dividing until a flattish irregular 

 sexual plant (the prothallium) a few millimetres in breadth 

 is produced. It bears sexual organs resembling those of the 

 ferns upon its edges or lobes; in some cases both kinds of 

 organs are on the same plant, while very commonly they 

 are upon separate plants. 



This class contains but one order (33, Eqitisbtaob^) of living 

 plants, including a single genus and twenty species. Among the 

 more well known are the common Horsetail (Equisetum arvense), 

 which sends up phort-lived, pale or brownish cone-bearing stems in 

 spring, and profusely branching green stems in summer (E. telmateia, 

 the Great Horsetail of Europe and our own Northwestern region, re- 

 sembles, but is larger than, the common Horsetail) ; the Woodland 

 Horsetail (E. sylvaticum), whose green cone-bearing stems branch 



