Seed ferns and cycads

The seed ferns (order Pteridospermales) and the cycads (Cycadales) are both very ancient groups. The seed ferns are known only from fossils dating from the mid-Devonian to early Cretaceous periods, 360 to 130 million years ago, when they had a worldwide distribution. About seven families can be identified, but the number of species is not known due to the fragmentary nature of the fossil record. The cycads, however, have living representatives, although they date back to the late Triassic period, 180 million years ago. They comprise 9 genera and about 75 species, all of which are now confined to the New and Old World tropics, whereas once they too were widespread.

Together the seed ferns and cycads make up the class Cycadopsida and are grouped as gymnosperms along with the other cone-bearing plants. They both have manoxylic wood—it is soft and spongy with wide parenchyma rays. Their leaves are large and frondlike and tend to branch pinnately (like a feather). In addition, their seeds are radially symmetrical (the same on all sides).

The male cones of the cycad Encephalartos frederici-guilelmii, of southern Africa, are formed from tight spirals of microsporo-phylls.

The seed ferns

The fossils of seed ferns are relatively common although incomplete. They reveal, nonetheless, that the leaves of these plants were large and fernlike (and consequently became broken at some stage during fossilization). It has also been found that the cambium in the stems formed large amounts of secondary xylem, unlike the cycads.

The plant parts have all been found but have not been joined together and consequently have been given separate scientific names. This has to a certain extent obscured the relationship between the various organs and made it even more difficult to build up a complete picture of the plants. This problem is compounded by the existence of a large number of different species. Enough has been discovered, however, to suggest that the appearance of seed ferns ranged from Lyginopteris, a scrambling plant with long thin stems and equally branched fertile fronds (sporophylls), to Sphenopteris, a large upright plant that resembled a tree fern.

Seed ferns evidently had neither cones nor flowers but produced seeds that were naked and not enclosed in an ovary. The presence of seeds places these plants among the gymnosperms and sets them apart from the ferns that they resemble so closely in appearance. The seeds were contained in leafy cupules on the ends of leaves or on special nonleafy branches (much like those on tree ferns). The seeds occurred either singly or in numbers up to about 70 in one cupule. These containers were shaped rather like a modern tulip flower, with the seeds on stalks inside them. It is thought that as each seed matured, the stalk elongated and carried the seed upwards to the mouth of the cupule, where it could more easily be dispersed.

Pollen sacs were also borne on the fronds. Without evidence, however, the method of pollination can only be guessed at, but it has been suggested that the female gametes were wind-pollinated. Nothing has been learned from any fossils of details of fertilization or embryo development.

Cycads

From what can be seen of their structure it is thought that the seed ferns formed an intermediate evolutionary stage between the true ferns and the conifers. Despite the fact that cycads show many primitive features, which suggest a close relationship with the seed ferns, they are considered to be intermediate between the seed ferns and the flowering plants, and some botanists believe that the flowering plants may have originated from the early cycads.

The cycad Encephalartos latifrondia indicates how palmlike these primitive plants are, except for their small stature—3 feet (91 centimeters) high at the most.

Most species of cycad are similar in appearance. The stem, or trunk, resembles that of a palm and can vary in height between species, up to about 60 feet (18 meters), as in the Australasian cycad Macrozamia hopei. Most, however, are only about 3 feet (91 centimeters) high and some species even have an underground stem, or tuber, such as Stangeria spp., from South Africa. The stem develops little secondary xylem but contains large amounts of parenchyma mixed with other conducting cells. It is usually unbranched and scarred with the bases of fallen leaves. The leaves, which grow in a crown at the top of the trunk, resemble fern or palm fronds and may vary in length from 10 feet (3 meters) in Cycas sp. to 2 inches (5 centimeters), as in Zamia pygmaea.

Male and female flowers occur on separate plants (they are dioecious) in the form of a cone at the top of the plant, up to about 3 feet (91 centimeters) long and 10 inches (25 centimeters) thick. The cones are formed in a spiral from fertile leaves (sporophylls). In the male flower the sporophylls are known as micro-sporophylls because pollen sacs containing the microspores, or pollen, grow on their underside. The microsporophylls may be up to 20 inches (50 centimeters) in length.

The sporophylls of the female cone are known as megasporophylls and are much smaller than the microsporophylls, being only 6 to 8 inches (15 to 20 centimeters) long. They are also conelike, except in Cycas, when they form a loose aggregation of sporophylls. The ovules, which may grow to the size of a hen’s egg, or larger, are formed in numbers from two to eight on the lower half of the megaspo-rophyll (as in Cycas) or may hang from under the sporophylls. Pollen released from the microsporangia is transferred to the ovules by the wind or by insects. Up to six months may pass before fertilization occurs, and the seed may take a year to mature.

Because the flowers are terminal, growth in male plants continues from an axillary bud at the base of the cone. The female flowers of most species of cycads are similar except in the case of Cycas, in which the apical meri-stem is unaffected by the flower and it continues its normal growth through the middle of the flower.

Cycads have adapted to survival in a dry climate by being xeromorphic—the long-lived ones live up to 1,000 years. The leaves have a thickened epidermis with sunken stomata. Apart from a long taproot, the roots are short and grow as coralloid masses (groups of short, thickened interwined roots), which contain symbiotic cyanobacteria, such as Anabaena.

The cycads have little economic value, although several species are grown as ornamental garden or house plants.

Pollination of cycads occurs when microspores, which have developed into pollen grains, are carried by the wind to the female megasporophyli. This body contains a gametophyte with two archegonia. The pollen grows a tube that fertilizes a gamete in one of the archegonia (this may take six months). The developing seed may take a year to germinate and is nourished by the starchy food-store of the gametophyte.