THE PTERIDOPHYTES 437 
develop female prothallia. Examples: Saluinia. Marsilea or the 
Water Shamrock, and Azolla. 
THe HorseEtTAILs 
The Equisetinee, commonly known as the Horsetails or Scour- 
ing Rushes, are perennial herbs with hollow, cylindrical, jointed 
and fluted stems, sheath-like whorls of united leaves and terminal 
cone-like fructifications. Their bodies contain large amounts of 
silicon, hence the name scouring rushes. Only 25 species exist 
today and all of these belong to the genus Equisetum. ‘These 
are believed to represent degenerate descendants of tree-like 
Horsetails, fossils of which have been found in coal-bearing rocks 
of the Carboniferous period. : 
In some species of Equisetum the fruiting cone is borne on the 
ordinary aerial stem, in others on a special’ stem of slightly 
different form. The spores are provided with ribbon-like 
appendages called elaters, which, being hygroscopic, coil and 
uncoil with increase or decrease in the amount of moisture 
present, thus aiding in the entanglement of the spores into small 
masses, and so preventing isolation, an advantage on account of 
the « dioecious character of the prothallia which they originate. 
Lire History or EquisETUM ARVENSE, A HorseTAIL 
Equisetum arvense, a frequently occurring species along railroad 
embankments but also in fields, may be taken as a type form of 
the group. Like other horsetails its life history involves an 
alternation of sporophyte and gametophyte generations of which 
the sporophyte is the more conspicuous. 
The spompkge consists of a subterranean, horizontally- 
branched, food-storing rhizome (main axis) bearing on its lower 
surface water- and mineral nutrient absorbing-roots and on its 
upper surface 2 kinds of jointed, hollow shoots—sterile and fertile. 
The sterile shoots are green and bear whorls of shortened, green 
branches from their upper nodes and scale-like leaves from their_ 
lower nodes, Theirs is the work of manufacturing food for the 
plant. The fertile shoots are nearly or completely devoid of 
chlorophyll and bear whorls of scale-like leaves at their nodes and 
terminate in a cone. The central stalk of this cone is a prolonga- 
