Cycads have the appearance of a cross between a tree fern and a palm but are related to neither. These slow-growing plants of the tropics and subtropics have unbranched trunks that grow more than 15 meters (50 feet) tall in a few species and have a crown of large pinnately divided leaves. Several of the approximately 100 known living species of cycads are presently facing extinction. During the Mesozoic era, now extinct gymnosperms known as cycadeoids were abundant. Cycadeoids bore a superficial resemblance to cycads but had very different reproductive structures and are not related.
Cycad life cycles are similar to those of conifers, except that pollination of cycads is generally brought about by beetles instead of wind. In addition, each sperm of cycads has from 10,000 to 20,000 spirally arranged flagella. Cycads are dioecious, and both the pollen stro-bili and seed strobili of some species are huge (e.g., more than a meter [3 feet 3 inches] long with a weight of over 220 kilograms [100 pounds]) (Fig. 22.12). The scales of seed strobili of some species are covered with feltlike or woolly hairs.
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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.