Introduction to Tissues and The Primary Growth of Stems

The body of an herb contains just three basic parts: leaves, stems, and roots (Fig.1). When the first land plants evolved about 420 million years ago, they were basically just algae that either washed up onto a shore or were left there as lakes and streams evaporated. They had no roots, stems, or leaves, and they just lay on the mud. As the shores gradually became crowded with such plants, some grew over others, shading them. Any plant which had a mutation that allowed it to grow upright into the sunshine above the others had a selective advantage. However, being upright is not easy: Elevated cells are out of contact with the moisture of the mud, so water must be transported up to them. Elevated tissues act as a sail and tend to blow down, so supportive tissue is necessary. Absorptive cells in the mud are shaded and cannot photosynthesize, so sugars must be transported down to them. Shortly after plants began living on land, distinct, specialized tissues and organs began to evolve.

FIGURE1: (a) The primary body of an herb like this geranium consists of roots, stems, and leaves; buds are located in the axil of each leaf and may grow to be either a vegetative branch or a set of flowers. (b) This Iris is also an herb and never produces wood or bark. All flowering plants are either broadleaf plants (dicots) like the geranium or plants with grasslike leaves (monocots) like the Iris. 

As these early populations of land plants continued to evolve and became taller, their stems functioned primarily as transport and support structures, as they do still. But stems of modern flowering plants have additional roles. They produce leaves and hold them in the sunlight, and during winter they store sugars and other nutrients, such as the sugary sap of maples. Stems may also be a means of survival: Underground bulbs and corms remain alive when above-ground leaves die. Stems of many species are a means of dispersal. They spread as runners or vines, or pieces break off and are carried by animals or water to new areas where they sprout roots and grow into new plants.

Although all flowering plants possess the basic structures of leaves, stems, and roots, these parts have been modified so extensively in some species they may not be recognizable without careful study. For example, cacti are often described as leafless, but they actually have small green leaves between 100 and 1000 /µm long (Fig. 2). A large, broad leaf would be selectively disadvantageous for these desert plants because the extensive surface area of such leaves gives up so much water to the dry air that the plant would desiccate.

Similarly, all flowering plants have stems, but in some they are only temporary, reduced structures. Orchids such as Campylocentrum pachyrrhizum and Harrisella porrecta consist of a mass of green photosynthetic roots connected to a tiny portion of stem; roots constitute almost the entire plant body (Fig. 3).

FIGURE 2 :This prickly pear (Opuntia) shows that one plant can have two types of shoot: the "pad" is the main shoot, and the spine clusters are highly modified branch shoots. One of the spinebearing branches has been stimulated to develop into the first type of shoot and become the earlike branches. The plant also has two types of leaves, small fleshy leaves on the young buds and spines on the axillary branches. 

The shoot becomes active only when flowers are to be produced. It has been hypothesized that this unusual body evolved because the ancestors of these species had roots that were more resistant to water stress than their stems were. Such plants could occupy drier habitats if either of two things happened: (1) Mutations were selected that caused stems and leaves to be more water conserving-this happened in most orchid species (Fig. 4), or (2) mutations occurred that enhanced the root's ability to absorb carbon dioxide and carry on photosynthesis. Although thousands of plant species are capable of withstanding harsh conditions, only a handful do so by being "shootless" and having photosynthetic roots. Mutations that permit this type of body may be either rare or generally detrimental.

FIGURE 3 :This orchid plant of the genus Polyradicion lindenii is composed almost completely of photosynthetic roots; only a small portion of shoot remains. Unlike most roots, these occur above ground and are green, being rich in chloroplasts. 

FIGURE 4: Most epiphytic orchids resist the stresses of temporary drying because their shoots are fibrous and have a thick cuticle composed of cutin and wax. 

Some plants in the bromeliad family are nearly rootless. In the coastal deserts south of Lima, Peru, fog is frequent but rain never falls. Because the soil is always dry, roots would be of little use. The Tillandsia straminea of that region is a small herbaceous vine that lies on top of the soil (Fig. 5). Plants absorb moisture through leaves made wet by fog, and they derive minerals from wind-blown dust that dissolves on the moist leaf surface. The plants are not anchored to the soil but roll over the coastal dunes as the wind blows them. Such a life style would be impossible for a tree or bush because large plants could not absorb enough water or minerals without roots.

FIGURE 5: These bromeliads are not rooted into the coastal sand dunes—a strong wind can blow them around. All water is absorbed from fog condensing on the leaves. 

In each plant described, leaves, stems, or roots have become highly modified by natural selection, permitting survival in unusual habitats. But in no species have any of these organs been completely lost evolutionarily. We must assume that the organ carries out some essential function. In the "shootless" orchids, the residual shoot is necessary for flowering and sexual reproduction. Cactus leaves are involved in the formation of the buds that produce the defensive spines. Although the roots of T. straminea neither absorb water and minerals nor anchor the plant, they may be essential for the production of critical hormones, as are the roots of other plants. In all cases, a careful analysis of many aspects of the plant's biology reveals how the structure and metabolism of a particular organ are adaptive, how modifications affect other plant parts, and how the organs have subtle functions not always obvious in more "typical" plants.

The flowering plants discussed in this and the next several chapters are formally classified as division Magnoliophyta (also frequently called division Anthophyta), but they are known informally as angiosperms. This group consists of about 235,000 species and is the largest division in the plant kingdom. Angiosperms have branched into two main lines of evolution: the dicots or broadleaf plants such as roses, asters, maples, and others, and the monocots such as grasses, lilies, cattails, palms, philodendrons, and bromeliads.

This chapter began by stating that the body of an herb contains three parts; it is necessary now to explain more precisely just what an herb is. Plant bodies are of two fundamental types: an herbaceous body, also called a primary plant body, and a woody body, known as a secondary plant body. An herb is a plant that never becomes woody and covered with bark; it often lives for less than a single year. Its tissues are primary tissues. In woody plants like trees and shrubs, the wood and bark are secondary tissues (Table ).