The Seed plants (Spermtophytae) are the most important group of land plants with about 250 000 - 300 000 species, but some also occur in water (e.g. seagrasses). Seed plants reproduce sexually by means of seeds and they produce pollen. The Spermatophytae comprise the following head groups:
All flowering plants, also called Angiosperms, how discrete their flowers might be in size or color, like in grasses and some trees. Angiosperms have seeds that are protected by a fruit
All Gymnosperms, to which the Conifers belong, known as cone-bearing needle trees. In Gymnosperms the seeds are 'naked'
The receptacle forms the base of the flower. The perianth often includes a number of brightly colored petals and smaller greenish sepals. The petals have the function to attract insects, while the sepals protect the flowerbud.
Not all flowers are as showy: grasses, for example, have mostly discrete greenish flowers arranged as ears or panicles into an inflorescence. Other flowers, i.e. those found in the family of the Composites, grow so close to each others that they give the impression to be one single flower.
In all flowering plants yet, the female reproductive organ is called pistil. It consists of an ovary interconnected by the stalk-shaped style to the stigma, the terminal portion. In the ovary ovules can be found which contain embryo sacs, the elements bearing the proper egg cells. In the male reproductive organs called stamina, distinction can be made between the filaments and thereabove the anthers. The anthers themselves consist of a number of loci where pollen is formed. Pollen carries the sperm cells, the true male reproductive cells. The transfer of pollen grains from the anther to the stigma of the pistil is called pollination. Fertilization, the fusion between sperm cell and egg cell, occurs in the ovary, where subsequently fruit and seed develop.
Nice examples of flowers are those of Petunia (Petunia hybrida), Yellow Loosestrife (Lysimachia vulgaris) and Common Medlar (Mespilus germanica) that often can be found in gardens.
Architecture of the flower
Flower organs, here in Petunia: (Left, whole flower; Middle and right, dissected flower). The petals (pe), the pistil (pi), the staminae or stamins (st) and the sepals (se). Sepals and petals together are also called the perianth (meaning literally: around the flower). In the stamins pollen grains (the carriers of the sperm cells, the true male sex cells) develop and mature and in the pistil the embryo sac with inside the egg cells (the actual female sex cells) are formed.
Flower of Yellow Loosestrife
The green pointed sepals are well visible inbetween the five big petals. The five dark stamins (e.g. 1 in the zoom view) arise above the petals. The greenish pistil (2 in the zoom view) is in the center
Common Meldar The Meldar has five large, white petals with in between five pointed, green sepals. There are many, brownish stamins. The green pistil has five distinct parts (inset)
The Gymnosperms
The Gymnosperms like needle trees (e.g. Pine tree, Pinus sylvestris) do not have flowers, but they bear cones (also called strobili). The cones are either male or female:
Life cycle of the Pine Tree
(Videomovie by Brittanica.com used with permission. Duration 2:17.
The female cones consist of scales that are arranged in a spiral around the branch. On each of these scales an embryo sac containing an egg cell can be found at either side of the middle vein.
The male cones, also arranged in a spiral but below the end of a young branch, carry at either side of the main vein of their scales a pollen sac in which pollen grains are formed. These plants are called Gymnosperms (Greek: gymnos = naked; Latin: sperma = seed, germ, sperm) because the pollen sacs and the ovules lay unprotected on the scales, as they are not surrounded by a leaf, like in the Flowering plants, also called Angiosperms (Greek: angeion = container, little urn).
Cones, reproductive scales and pollen in Gymnosperms (here: Pine)
A. The male cones are arranged in a spiral below the top of young branches. Also the scales within a cone are distributed along a spiral.
B. The scales of the male cone carry two oval-shape pollen sacs.
C. Microscopical view of a stained longitudinal section through a male cone. (zoom)
D. Microscopical view of a stained section through a male scale and a pollen sac. 1=Extension of the male spore-carrying pollen sac (microsporophyl = 2n), 2 = wall of the male pollen sac (2n), 3 = pollen grains (contain haploid cells, thus 1n). When the wall of the pollen sac dries it cracks open and pollen grains can be dispersed by the wind (anemophily). (zoom)
E. Pollen grains with lateral air bags orbladders
F. Microscopical view of a stained slide of a pine pollen tube. 1 = Air bags or air bladders, 2 = Generative cell with generative nucleus, 3 = pollen tube, 4 = tube nucleus; zoom
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Drawings: Lidwien van der Horst
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