Plant Structure

Bryan G. Bowes, James D. Mauseth

• Publisher: CRC Press
• Year: 2008
• ISBN:
  9781482261264
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

This book is a fundamental guide to understanding plant structure offering plant scientists, plant biologists and horticulturalists in practice, academic life and in training. It includes a combination of concise scientific text and superb color photographs and drawings, focusing on structure at anatomical, histological and fine structure levels.

CHAPTER 1
Introduction

THE ASSORTMENT OF LAND PLANTS

Flowering plants, or angiosperms (1, 2), dominate large areas of the land surface and represent the climax of vascular plant evolution. They occupy a wide range of habitats and about a quarter of a million species have been recognized so far. However, many more, particularly from tropical regions, await scientific description. Angiosperms are very diversified in their form, and range in size from a few millimetres in diameter in the aquatic Lemna (3) to 90 m or more in height in Eucalyptus (4). Some complete their life cycle in less than 2 months while some specimens of Quercus (oak) may live nearly a thousand years (5).

Flowering plants provide the vast majority of those consumed by humans (6, 7) or utilized for domestic animal fodder. Likewise, angiosperms provide various very important commercial hardwood timbers such as Acacia, Carya, Eucalyptus, Fagus, Juglans, and Quercus (4, 8, 9), fibres (e.g. Corchorus, Linum; 10) and drugs (e.g. Papaver, Coffea; 7). Most decorative garden plants are grown to provide floral displays (1, 2).

Although the flowering plants are now dominant in many habitats, remnants of earlier evolved vascular plant groups are still present in the flora. There are about 700 species of gymnosperms whose seeds are naked (11); these are mostly conifers (12) but there are also some 289 tropical/sub-tropical cycad species (11) and a few others. Their naked seeds distinguish them from the flowering plants where the seeds are enclosed within a fruit (13). The conifers dominate the vast tracts of boreal forest which occur in North America and northern Europe and Asia, and many conifers provide very valuable softwood timbers for a multitude of purposes such as construction, paper pulp, and fencing. The spore-bearing ferns and their allies (14–16) number about 12,000 species.

As well as over 260,000 species of vascular plants, the land flora includes the nonvascular, spore-bearing bryophytes (17, 18). These small plants comprise about 1500 species of moss (18) but far fewer liverworts (17) and hornworts. They lack cuticular covering to the epidermis and are usually confined to moist locations. Hornworts and liverworts are often simply-organized thalloid structures without leaves, but mosses (and some liverworts) are more complex and have leafy green shoots.

The stems of many mosses and a few liverworts show a central strand of tissue, apparently concerned with the movement of water and soluble foodstuffs. However, except in a few taxa such as Polytrichum (19), this does not have the structural complexity of the xylem and phloem tissues (20) of vascular plants. The latter tissues are concerned with the rapid, long-distance transport of water and soluble foodstuffs (21).

Lichens are not true plants but rather symbiotic associations of fungi and algae; these however, often show a complex plant-like form (22).

TRANSPIRATION AND TRANSLOCATION IN VASCULAR PLANTS

The root system absorbs water, together with dissolved mineral salts, from the soil. This passes across the cortex and endodermis of the young root to the central xylem (23). The dead tracheary elements of this tissue have strong thickened walls (20) and their lumina are filled with columns of water moving upwards into the shoot (21).

This transpiration stream is powered by the evaporation of water vapour from the shoot surface, and mainly occurs through the stomata in the leaf epidermis. These small pores (24) normally remain open in the day and allow the entry of carbon dioxide, which is essential for photosynthesis in the green foliage. The sugars thus elaborated are translocated (21) in solution in the living sieve elements of the phloem (20) to the stem and root where they are either stored (23, 25) or metabolized.

GENERAL MORPHOLOGY OF ANGIOSPERMS

The young shoot of the generalized flowering plant (26) bears a number of leaves and normally a lateral bud occurs in the axil of each leaf. The leaf is attached to the stem at the node, while the internode lies between successive leaves. The leaf is usually flat and often is borne on a leaf stalk (petiole, 27). In a horizontal leaf the adaxial surface (which was nearest to the shoot apex while within the bud, 28) lies uppermost and the abaxial side forms the lower surface.

A simple leaf may be dissected or lobed, and a compound leaf shows several leaflets (29); leaflets do not subtend axillary buds. In the lamina (leaf blade) a network of veins is present (30) which links to the vascular system of the stem. The axillary (lateral) buds may remain dormant but normally develop into side shoots, or form flowers. At the base of the main stem the cotyledons (first leaves formed in the embryo; 31) demarcate it from the hypocotyl; the latter represents a transition zone between stem and root.

Until recently, angiosperms were believed to consist of just two groups: the dicotyledons (dicots) and the monocotyledons (monocots). However, cladistic studies of DNA and other features have proposed that this fundamental diversification of angiosperms did not occur until after about eight small basal orders (containing less than 3% of all angiosperm species) had already become distinct. Four of these basal orders are now grouped together as the ‘Magnoliids,’ another is distinct as the water lilies (Nymphaeales). The rest of the angiosperms are divided into the monocots (containing about 22% of all angiosperm species), a separate order Ceratophyllales, and finally the ‘eudicots’ (with about 75% of all angiosperm species). In most structural features these all still form two fundamental groups: those with monocot-like structure (the monocots, of course) and those with dicot-like structure (almost everything except the monocots). There is no term to describe ‘all the angiosperms other than monocots’ other than the old term ‘dicot;’ we will th...