Simple, plain in appearance, and without true leaves, the psilophytes are not plants of extreme beauty, although some are often cultivated, but they are unusual and often quite unique. They all have vascular systems, but lack seeds and roots. Some species have the odd characteristic of growing off of another plant and in turn having fungi growing in its own clefts.
The Psilotum, or whisk fern, is one of the psilophytes still living today, being found in damp, rich soils of tropical or subtropical regions, such as Hawaii, Florida, Texas, or the islands of the Caribbean. Many times the Psilotum plants grow on trees or other plants, but not as parasites. In Japan, certain forms of the species Psilotum nudum are very popular for the subjects of books and for cultivation.
About two feet in height, Psilotum plants have upright, aerial stems coming up from an underground rhizome. With scales arranged spirally around them, the green, dichotomous stems (stems that fork out continuously in two ways, similar to the branches of trees) are covered in a waxy material called cutin and are photosynthetic. Three-lobed sporangia are produced on the stems of mature plants. The rhizome, which absorbs nutrition from the ground and anchors the plant, is covered in hairlike rhizoids and, like the stem, has a cutinized epidermis. However, this amount of cutin is so large that even after internal tissues die and decompose the rhizome becomes hollow and stays in the ground. Often found in the rhizome of Psilotum plants are endophytic fungi, which grow in the cells of the rhizomes and benefit the plant by breaking down food. Both the upright stems and rhizomes have simple vascular tissue.
A native of Australia, New Caledonia, Tasmania, the
Polynesian islands, New Zealand, and islands of the Philippines, the one species
of Tmesipteris is the only other psilophyte around today and is usually
endophytic, meaning that it grows on other plants. Half-inch long, leaf-like
structures covering two stems, which either
Existence of the Psilotum and Tmesipteris plants continues by the alternation of the gametophyte and sporophyte generations. Inside the sporangia of the plant, spore mother cells go through meiosis to form haploid spores, which are dispersed over a wide area after the cracking open of the sporangia. Following germination, the spore slowly develops into a cylinder-shaped gametophyte beneath the ground and is one to fifteen millimeters long and two millimeters in diameter. Because the gametophyte has no chlorophyll, it is saprophytic, meaning that it obtains food by absorbing nonliving, organic matter from around it, and this task is probably lightened by the help of the endophytic fungi residing with it. From the antheridia of the gametophyte swim multiflagellate sperm to fertilize the egg in the archegonia, where a zygote then slowly develops into a sporophyte. The sporophyte produces above-ground stems and becomes a plant that independently makes its nutrition.
Although most of the psilophytes no longer exist today, some fossils and descriptions of extinct plants still remain, one description being of Psilophyton made in 1858 by Sir William Dawson. These plants were under three feet tall, and had dichotomous stems and underground rhizomes like Psilotum plants, but the stems were covered with spiny protrusions. Tips of branches were coiled and circinate (unrolling as they develop), and sporangia were produced in pairs. Descriptions were also made in the early 1900's of similar plants, which were the Rhynia, Horneophyton, and Asteroxylon, living in silicified peat deposits near Rhynie, Scotland.
Even though they are plain and usually bare, the psilophytes that still exist in parts of the world today add diversity to the vegetation of the earth. They have been given purpose and design by a Creator and are worthy of study.
Bold, Harold C. The Plant Kingdom. Englewood Cliffs, New Jersey:
Prentice-Hall, Inc., 1970.
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