Gnetum Study Notes

Gnetum Systematic Position: Class: Gnetopsida Order: Gnetales Family: Gnetaceae Genus: Gnetum Morphology: Gnetum is the most advanced genus among gymnosperms. The plant body is a sporophyte differentiated into root, stem, and leaves. The root is profusely branched taproot. The stem bears two kinds of branches: dwarf shoots of limited growth and long shoots of unlimited growth. The leaves resemble dicot leaves. The foliage leaves are present only on dwarf shoots and long shoots contain cup-like pairs of reduced scale leaves. Reproduction: Gnetum is dioecious, but bisexual strobili also occur in Gnetum africanum. Reproductive organs are called flowers, which are arranged in inflorescence. Both male and female inflorescences are axillary in position. The male inflorescence consists of a long stalk ending in a cluster of flowers. Flowers arise in the axils of bracts. The bracts are cup-like forming a collar around the axis. There are 10-35 flowers in each strobilus. Male Strobilus: The male strobilus has a short stalk arising in the axil of conical bracts. 10-25 whorls of bracts are present at short intervals in the axis. Bracts at each node fuse to form a cup-like collar. On each collar, many male flowers are arranged into 5-6 whorls. Above the male flower whorl, a whorl of abortive female flowers or ovules is present. Male Flowers: A male flower consists of a coherent structure called perianth, which encloses a unilocular anther. When the flower matures, the wall of the anther elongates, and the anther is pushed out of the perianth sheath. Microsporangium: Each microsporangium develops from one or more hypodermal archesporial cells, dividing periclinally and forming primary sporogenous cells. The sporogenous cells divide to produce microspore mother cells. Each microspore mother cell undergoes meiosis and forms a haploid microspore. Female Strobilus: Like the male strobilus, the female strobilus has several collars arranged one above the other at short intervals. Each collar has a whorl of 4-6 ovules. Female Flowers: A female flower consists of a short stalk and an ovule (megasporangium). Each ovule has a nucellus protected by 3-layered integuments. The innermost integuments elongate to form a long micropylar tube. A rudimentary pollen chamber is present at the tip of the nucellus. The main axis of the female strobilus has 12-14 vascular bundles. Megasporangium: In the hypodermal region of the nucellus, 1-4 archesporial cells differentiate. These cells divide and form primary parietal and primary sporogenous cells. The primary parietal cells divide repeatedly, forming massive nuclear tissue. The sporogenous cell functions as the megaspore mother cell. As no cell walls are produced in the megaspore tetrad, it is used in the development of the female gametophyte. Gametophyte: Tetrasporic and tetraloculate megaspore develop into male and female gametophytes, respectively. Development of Male Gametophyte: The microspore (pollen grain) is the first stage of male gametophyte development. It is uninucleate, covered by a thick exine. The microspore nucleus divides to form 2 daughter nuclei. These nuclei are called prothallial and generative nuclei, with a tube nucleus. The generative nucleus divides to form a body cell and a stalk cell. Microspores are carried by the wind to the pollen chamber, forming a pair of sperm nuclei. Development of Female Gametophyte: The megaspore is the first cell of the female gametophyte. Free nuclear division occurs, producing a large central vacuole. The female gametophyte elongates towards the chalazal end. Fertilization: Microspores germinate to form pollen tubes, growing through the nucellus. The tube nucleus and generative cell divide to form 2 male cells. Only one of the male cells fuses with the egg to form a zygote. Endosperm: Endosperm development begins before fertilization. The lower part of the female gametophyte becomes cellular before fertilization. After fertilization, the upper part becomes cellular. Embryo Development: The zygote grows towards the endosperm. Primary suspensor tubes coil around each other. The embryo increases in size, forming cotyledons, a root cap, and a shoot tip.