Zon 
length of seventy-one feet, being two feet in diameter at one end 
and tapering only four inches in that distance. Others were 
quite conical. One French specimen was six feet in diameter at 
the base, and tapered to a foot in diameter eighteen feet above the 
base. The curious and varied appearance of the sigillarias are 
well shown in the right hand third of the transparency. 
5. Horsetail Group 
Modern systematic botanists often classify the Horsetails or 
Scouring Rushes (/:quisetim) as “ fern allies,” but they are really 
the survivors of a great independent group, which goes back to 
Devoman times and furnished some of the dominant arborescent 
forms during the later Paleozoic. To this group the name Arthro- 
— 
phytais given. They are characterized throughout their age-long 
lustory by having articulated and prevailingly ribbed stems with 
the relatively small leaves arranged in whorls (verticillate) at the 
nodes. 
In the earliest forms (Pseudoboriia, Protocalamaria, some spe- 
cies of Sphenophylluit) the leaves are somewhat less reduced and 
—, 
compound, but with the passage of time they become progressively 
smaller, and in the modern forms (/¢quisetum) the green stems 
and branches have taken over the photosynthetic function. Their 
spore producing organs are in the form of cones, although these 
show considerable morphological and histological variation. Some 
produce but one kind of spores (homospory), although the major- 
ity in the past produced both large and small spores (/heterospory). 
Vhe Arthrophyta attained their maximum differentiation and 
size during the later Paleozoic, and since the Triassic they have 
been represented by only the single genus, Equisetum. They fall 
rather naturally ito two main groups—the Sphenophyllae, repre- 
sented by Sphenophyllum in the transparency, and the Calamariae, 
which includes Pseudoborma and Calamites of the transparency, 
as well as the more modern [quisetums. 
The Sphenophyllums stand rather by themselves in the Horse- 
tail group, since they show some features that remind one of the 
Lepidophytes. They range as impressions of slender jointed 
ribbed stems with whorls of mostly wedge-shaped small leaves 
1 Other names are Articulata, Calamophyta, and Sphenopsida. 
