EQUISETACE^E. 161 



E. limosum and E. arvense. If this be so, it is not unlikely to 

 occur in Britain. E. littorale has the general habit of the forms of 

 E. arvense which have branched fertile stems, but the rootstock is 

 angular and hollow, and there is no ring of thickened tissue in the 

 stem ; the branches also are generally hollow. 



E. limosum is a variable plant, but the variations run too much 

 into each other to be separable into varieties; even the two forms 

 which I have admitted as varieties are most difficult to define, and 

 may very possibly be merely states of the plant due to external 

 circumstances. I have, however, retained them, as they are generally 

 accepted in this country, and were considered distinct species by both 

 Linnaeus and Fries. 



Water Horsetail. 



Section IV.— HYEMALIA. A. Brawn. 



Stems all similar, persisting, green, rough to the touch, branched 

 or unbranched. Stomata sunk in depressions so as to be below the 

 general surface of the epidermis. Sheaths with persistent or 

 deciduous teeth. Branches usually solitary, rarely in whorls, often 

 absent, with a central cavity. Fertile stems differing from the sterile 

 ones only in being terminated by a spike, which is perfected in autumn 

 or late summer. Spike mucronate or apiculate, usually black. 



SPECIES VII.— E QUISETUM HYEMALE. ' Linn: (auct. plur.) 



Plates 1894 and 1895. 



Stems all similar, sub-evergreen, solitary or several together from 

 each node or extremity of branch of the rootstock rather stout or 

 rather slender, with a central hollow off or \ its diameter, with 8 to 34 

 rather shallow furrows, separated by subobtuse edges, which are not 

 furrowed on the back, and are rough, with small prominent tubercles 

 arranged in one stripe on each ridge, dull dark green. Sheaths 

 cylindrical, applied to the stem or slightly widened upwards, at first 

 pale green and concolorous, then with a black band at the apex and 

 afterwards another at the base, afterwards wholly black, ultimately 

 white with a black band at the base and a narrower one at the apex ; 

 the lower ones permanently black ; each of the portions of the sheath 

 which corresponds to one of the teeth with a narrow shallow furrow 

 down the centre, and another similar furrow on each side, midway 

 between the central furrow and the great furrow which extends 

 (between the teeth) from the apex to the base of the sheath ; teeth 



VOL. XII. Y 



