66 BOTANY. 



burst open. This species is common in the United States. Ceterach 

 officinarum {ßg: 49'') ; C leptophylla {fig: 48.) 



Order 9. EauisEXACE^, Horse tails. Stem striated, hollow, usually 

 branched, containing much silica in its composition, articulated, the joints 

 being separate, and surrounded by a membranous -toothed sheath. There 

 are no true leaves, green-colored branches having a straight vernation, 

 occupying their place. The cuticle exhibits a longitudinal series of 

 stomata. A spiral structure is observed in some of the vessels. Reproduc- 

 tive organs collected into cones ; spore-cases (thecas or sporangia) attached to 

 the lower surface of peltate polygonal scales, and opening by an internal 

 longitudinal fissure ; spores in the form of rounded cells, surrounded by two 

 clastic club-shaped, hygrometric filaments, or elaters. Plants, with simple or 

 l)ranched stems, the branches being jointed and placed in whorls at the 

 articulations of the stem, each wliorl consisting of as many branches as there 

 are teeth in the sheath. Found in ditches, lakes, and rivers, in various parts 

 of the world. 



From the quantity of silicic acid contained in them, some of the species 

 of Equisetum are used in polishing woods and in scouring utensils. The 

 spiral filaments Avhich surround their spores arc interesting objects under 

 the microscope, exhibiting marked movements according to the moisture 

 or dryness of the atmosphere around them. The stomata are arranged in 

 lines on the cuticle. In Equisetum hyemule, often called Dutch rushes^ 

 the silicious stomatic apparatus is well seen after the action of nitric acid on 

 the stem. There are regular rows of tubercles of a silicious nature, in each 

 of which is a transverse fissure, and at the bottom of the fissure a stoma is 

 placed, with its opening at right angles to that of the tubercle. Each 

 portion of the stoma lias a pectinated (comb-like) appearance. The distinctions 

 between the species of Equisetum are founded on the nature of the fertile 

 and barren stems, the number of strife or furrows, and the number of teeth at 

 the articulations. 



There is but a single genus Equisetum, represented in North America by 

 numerous species. One of these is Equisetum hyemale ; another is E. 

 limosum {pi. 54, fig. 59). 



Section B. Phanerogamous Plants. 



Class 2. Monocotylednnes, Juss. Endog'encB, D. C. Aniphibrya, Endl. 



In this great class the plants have a cellular and vascular system, the 

 latter consisting partly of elastic spiral vessels. The woody stem is usually 

 more or less cylindrical, simple, and unbranched. There is no true 

 separable bark, no concentric zones, and no true pith. The wood is 

 endogenous, i. e. increases by additions which first tend towards the centre, 

 and then curve outwards in an interlacing manner towards the circum- 

 ference, where much hard ligneous matter is deposited, so as to make the 

 exterior the hardest part. The development of the stem usually takes 

 66 



