The plants of members of this phylum vary greatly in size, shape, texture, form, and longevity. The phylum includes, for example, the tiny duckweeds that may be less than 1 millimeter long, all the grasses and palms, many aquatic and epiphytic plants, and most shrubs and trees, including the huge Eucalyptus regnans trees of Tasmania that rival the redwoods in total volume.
A few flowering plants are parasitic. Dodders, for example, occasionally cause serious crop losses as they twine about their hosts and, by means of haustoria (shown in Fig. 5.15B), intercept food and water in the host xylem and phloem. Broomrapes also parasitize a variety of plants, as do mistletoes. Mistletoes produce some chlorophyll and depend only partially on their hosts for food. Still others, such as the beautiful snowplant (Fig. 23.2) and some of the orchids, are saprophytes (i.e., their nutrition comes mostly from the absorption in solution of dead organic matter). The vast majority of flowering plants, however, produce their food independently through photosynthesis.
Like the gymnosperms, the angiosperms are het-erosporous (produce two kinds of spores), and the sporo-phytes are even more dominant than in the gymnosperms. The female gametophytes are wholly enclosed within sporo-phyte tissue and reduced to only a few cells. At maturity, the male gametophytes consist of a germinated pollen grain with three nuclei.
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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.