636 EQUISETACE^. 



numerous spermatozoids ; the macrospores form a more or less rudi- 

 mentary prothallus, with one or more Archegonia. Ferns have a 

 monoicous' prothallus ; Equisetums have a dioieous prothallus — a 

 small one forming Antheridia, a larger Archegonia. In Ophioglossums, 

 which some authors put as an order distinct from ferns, the prothallus 

 is not green, is produced under ground, and bears both Antheridia 

 and Archegonia. 



Order 215. — Eqtjisetace^, the Horse-tail Family. Stem 

 striated, hollow, usually branched, containing much silica in its com- 

 position, articulated, the joints being separable, and surrounded by a 

 membranous toothed sheath, formed by scales, which are equivalent 

 to leaves (fig. 139, p. 73). There are no true leaves, green-coloured 

 branches, having a straight vernation, 'occupying their place; soms' 

 stems are barren, others fertile. The cuticle exhibits a longitudinal 

 series of stomata. A spiral structure is observed in some of the 

 vessels. The stems S,re also traversed by air canals (fig. 140, p. 74). 

 Reproductive organs collected into cones ; spore-cases (thecse or spo- 



MX 



Fig. 877. rig. S78. Kg. 879. Fig. 880. 



rangia) attached to the lower surface of peltate polygonal scales (fig. 

 877), and opening by an internal longitudinal fissure (fig. 878) ; 

 spores in the form of rounded cells, surrounded by 4 elastic, club- 

 shaped, hygrometric filaments or elaters (figs. 879, 880), formed by 

 the breaking up of the outer coat of the spore in a spiral manner. 

 The spore in germinating produces a green prothallus, on which 

 Antheridia (containing spermatozoids) and Archegonia are formed, the 

 latter producing the leafy stems.^ — Plants with simple or branched 

 stems, the branches being jointed and placed in whorls at the articu- 

 lations of the stem, each whorl consisting of as many branches as 

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

 in various parts of the world. In South America, Gardner measured 



Figs. 877-880. Reproductive organs of Equisetum, to illustrate tlie natua-al order Equi- 

 setaceae. Fig. 877. A peltate or polygonal scale, e, taken from tlie terminal cone-like 

 fructification of an Equisetum. c, Thecffi or spore-cases arranged in a verticil on the imder 

 surface of the scale. «, Stalk by which the scale is attached to the axis. Fig. 878. c. 

 Spore-case seen on ifsinner surface, with the slit or opening by which the spores are dis- 

 charged. Fig. 879. A spore, s, with four clavate filaments roUed up in a spiral man- 

 ner around it. Fig. 880. Spore, s, with the filaments, which are clavate at their ex- 

 tremities, unrolled. These filaments or elaters are hygrometric, and move about under the 

 influence of moisture. 



