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The
Angiosperms
(Campbell 6th Ed. 606-613 & 783-794; 7th Ed. 598-606
& 771-780)
The angiosperms or flowering plants comprise a diverse assemblage of nearly a quarter million different species. There are two subclasses of angiosperms, the Monocotyledoneae (monocots) represented by some 65,000 species and the Dicotyledoneae (dicots) represented by some 170,000 species. All monocots are herbaceous and include such economically important plants as grasses (e.g. corn, wheat, barley), lilies, daffodils and palms. The dicots include both woody and herbaceous members. Woody trees such as elm and oak, woody shrubs such as dogwood and herbaceous plants such as beans, geraniums and rapeseed belong to this subclass.
The basic differences between these two subclasses of angiosperms are outlined in Campbell 6th Ed. 721 & 794 Fig. 38.14; 7th Ed. 602-603 & 780 Fig. 38.10.
Angiosperms are found in a wide range of habitats. The variability stems from
an effective means of sexual reproduction. Like the gymnosperms, the angiosperms
produce seeds and possess pollen
tubes to facilitate the transfer of sperm. However, unlike the gymnosperms,
the angiosperm ovules and microsporangia develop in certain highly modified fertile
leaves (technically sporophylls) of a flower.
The angiosperm flower is considered to be a reproductive shoot (stem and highly modified leaves) which exhibits determinate growth (their apical meristem produces a set number of flower parts and then stops growing). Depending on the species, the flowers may be single or clustered into an aggregation of flowers termed an inflorescence.
Typically, the flower parts arise from the enlarged receptacle of the flower stalk (the stem). These floral appendages are in four distinct series. The first (outermost) series of floral appendages is composed of small, green, leaf-like sepals. The sepals are collectively termed a calyx. The sepals enclose and protect the other floral parts during the bud stage of flower development.
The second series of floral appendages is the petals. The petals are collectively called a corolla. In some angiosperms the petals are reduced or absent.
The next two series of appendages are highly modified fertile leaves (sporophylls). The stamens - collectively the androecium - are microsporophylls. The stamen consists of a slender stalk, or filament, upon which is borne a two-lobed anther containing four microsporangia.
The innermost series of floral appendages is the pistil. The pistil is a carpel or a whorl of fused carpels. The carpels - collectively the gynoecium - are ovule producing megasporophylls that are folded lengthwise, enclosing one or more ovules. A pistil composed of one carpel is called a simple pistil. A pistil composed of fused carpels is called a compound pistil. In both, the pistil is differentiated into a lower part, the ovary, which encloses the ovules, and an upper part, the stigma, which receives the pollen. In many angiosperm flowers an elongated structure, the style, connects the stigma with the ovary.
A floral diagram represents a cross-section of a flower as it would appear if all parts were at the same level. The various floral appendages are represented in diagrams by standardized symbols (refer to lab manual). Most flowers are constructed upon a definite numerical plan. In monocots the flowers usually have a numerical plan of three or multiples of three (e.g. 3 sepals, 3 petals, 6 stamens). Dicot flowers are usually constructed on a numerical plan of four or five or multiples of these. The numerical plan of the flower is most evident in the sepals and petals and in some flowers this is carried through to the stamens. This feature of construction may, but often does not apply to the carpels.
