264 PHYLUM X. CALAMOPHYTA 
in height, but in certain tropical species attaining a 
length of 10 meters or more. Among the better known 
are the Common Horsetail (Hquisetum arvense), which 
sends up short lived, pale or brownish cone-bearing stems 
in spring, and profusely branching green stems in sum- 
mer (Z. telmateia, the Great Horsetail of Europe and our 
own Northwestern region, resembles, but is larger than, 
the Common Horsetail); the Woodland Horsetail (E. 
sylvaticum), whose green cone-bearing stems branch 
profusely after fruiting, and persist all summer; and the 
Scouring-rush, called also Dutch Rush (E. hiemale), 
with green, branchless stems which produce cones, and 
survive the winter. The genus Equisetum originated in 
the Paleozoic period, and so is very old. Some of its 
species have become extinct, as is the case with several 
related genera. 
473. The Old Calamites (Class CALAMARINEAE) were 
Paleozoic plants whose sporophytes were 
often trees, with hollow, jointed stems, 
whose collateral vascular bundles allowed 
an increase in diameter by the develop- 
ment of a cambial zone. The leaves 
were separate, narrow, and whorled at 
= ino A es the joints of the stem. The heterospores 
of Old Calamite were borne in terminal cones composed 
of whorls of sporophylls, each bearing one or more spo- 
rangia. Only fragmentary fossils of these plants are 
known. | 
Laboratory Studies. (a) Collect in early spring a number 
of cone-bearing stems of the Common Horsetail. Note the 
joints (nodes), bearing whorls of united flat leaves, and the 
cone, composed of whorls of shield-shaped leaves (sporophylls). 
Split the cone and stem and note that the latter is hollow, with 
closed nodes. 
(6) Carefully dissect out a single sporophyll from the cone, 
