Horsetails and scouring rushes belong to the single remaining genus (Equisetum) of a phylum of several different orders that flourished in the Carboniferous period about 300 million years ago. At that time, some members of this phylum, like the ancient club mosses, were treelike (Fig. 21.14) and grew over 15 meters (50 feet) tall, while others were vinelike. Some were similar to present-day horsetails in having leaves in whorls (Fig. 21.15), jointed stems with internal canals, and sporangiophores with sporangia hanging down from the rims.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.