MODULE I UNIT 3 THE PHYLUM CYCADOPHYTA -THE CYCADS The Cycads are the slow-growing plants of the tropics and subtropics and have unbranched trunks that grow more than 15 meters tall in a few species and have a crown of large, pinnately divided leaves (Fig. 12). Cycads have a number of distinctive features. They are the only gymnosperms to produce large compound leaves. They branch rarely, if at all, although they have secondary growth, the wood is different from the wood of conifers because it has a lot of parenchyma in the xylem. Cycad life cycles are similar to those of conifers, except that pollination of cycads is sometimes brought about by beetles instead of wind. Each sperm of cycads has from 10,000 to 20,000 spirally arranged flagella. They are dioecious, and both the pollen strobili (Fig. 13A) and the seed strobilus (Fig. 13B) of some of some species are huge and long with a weight of over 220kg. The seeds of many cycads are covered with a fleshy, brightly coloured seed coat to attract animal dispersers. Figure 12: A Cycad Image source: https://medium.com/think-with-me/cycads-93ad7aa0c822 1 A B Figure 13: A male cycad with a strobili B. A female cycad with a strobilus Image source: http://www.indefenseofplants.com/blog/tag/cycad https://edition.cnn.com/2019/08/22/uk/tropical-plant-uk-intl-scli-gbr/index.html They are palm-like plants, found mainly in tropical and sub-tropical regions. Living cycads comprise eleven (11) genera, with about 140 species. They are mostly large plants, reaching 18 metres or more in height. The stem of a cycad is unbranched, erect, stout and palm-like, with a crown of pinnate leaves arranged spirally around the apex. Small, dry, scale-like leaves alternate with the green pinnate leaves. The plant possesses a well formed, long (deep) taproot system. A common name for some cycads is ‘sago palms’. Cycads are the largest group of gymnosperms after conifers. They exhibit true secondary growth from a vascular cambium; the central portion of their trunks consists of a great mass of pith cells. Common examples include: Zamia pumulia, Cycasrevoluta,Cycascircinalis,Encephalartosferox, etc. Reproduction in Cycads Cycads are dioecious, that is, male and female flowers borne on two separate plants. The male flower is a cone, borne at the apex of the stem, which then grows by a lateral bud. The male cone consists of a collection of microsporophylls (stamens) spirally arranged round the axis. Each sporophyll is in the form of a scale, narrow below and broadened above. Its undersurface bears 2 several pollen sacs or microsporangia grouped in sori. There are usually 2 to 6 pollen sacs in each sorus. Each pollen sac contains many pollen grains or microspores. There is no proper female flower in cycads. The plant bears near its apex a rosette of megasporophylls which do not form a cone but are alternately arranged with the leaves. The carpel in gymnosperms is always open, and the ovules are borne freely exposed, on the two margins of the carpel. The ovule grows considerably even before fertilization. A megaspore mother cell is produced within the nucellus of the ovule. This divides into a row of four megaspores. Only one megaspore is functional, the other three disintegrate. The functional megaspore rapidly divides and gives rise to the female gametophyte. Pollination and fertilization Pollen grains are carried by wind and fall on the micropyle end of the ovule. Mucilage secreted by the latter draws the pollen grain into the pollen chamber. The pollen grain divides and forms the male gametophyte (prothallus cell), generative cell and tube cell. The tube cell elongates into a long, branched pollen-tube which enters the archegonial chamber, the membrane between the two chambers having broken down. The growth of the pollen tube significantly destroys nucellar tissue in most cycads, because it is a sucking organ (haustorium) absorbing food from the nucellus, rather than a sperm carrier. The generative cell divides into a sterile stalk cell and body cell – the latter forming into two large spermatozoids. The pollen tube bursts at the apex and releases the sperm cells, which swim to the archegonium and enters the egg cell. Fertilization is effected when the male sperm nucleus fuses with the egg nucleus. The interval between pollination and fertilization is about four months. Insects also play an interesting role in the pollination of cycads. For example, weevils of the genus Rhopalotria carry out entire life cycles upon and within the male cones of Zamia pumulia and also visit the female cones. Beetles of several groups are most frequently associated (with male cones); and less frequently with female cones of several cycads. Also, pollen eating bees have also been long associated with cycads. 3 Economic Importance of Cycads They are often highly toxic, with both neurotoxins and carcinogenic compounds abundant as plant metabolites They harbor cyanobacteria and make important contributions of fixed nitrogen to the soils where they occur The leaves of Cycas circinalis are used to make hats, baskets and mats Bark, leaves, seeds and resin of cycad plants are known to possess medicinal and therapeutic values. For example, bark and seeds of C. circinalis are ground to paste with coconut oil and used as poultice for sores and swellings; resin obtained from C. rumphii are applied to malignant ulcers; juice of tender leaves of C. circinalis are useful for flatulence and vomiting The general Cycas, Dioon, and Zamia are widely propagated as ornamental plants, either outdoors or in greenhouses. The stem pith can yield edible starch. 4 .