TIPOS DE ICTERICIA

Advantages of Dispersal of fruits and seeds

Shoot System-Stems, Leaves, Flower and Fruits 123

ix. The floral leaves consist of sepals, (calyx), petals (calyx), stamen (androecium) and carpels (gynoecium).

x. A fruit is the sturcture which develops from the ovary after fertilisation xi. Fruits can be classified as simple, aggregates and multiple fruits.

List at least four functions each of the following i. Stems (2 marks)

ii. Leaves (2 marks) iii. Calyx (2 marks) iv. Corolla (2 marks)

Idodo - Vmeh (1996),

Sarojini T. Ramalingam (2001)

Page

1.0 Introduction 125

2.0 Objectives 125

3.0 Herbs 125

3.1 Herbaceous Monocotyledonous Stem 125

3.2 Herbaceous Dicotyledonous Stem 125

3.3 Shrubs 126

3.3.1 Woody Dicotyledonous Stems of Shrubs 126

3.4 Trees 126

3.5 Ephemerals 127

3.6 Annual Plants 127

3.7 Biennial Plants 127

3.8 Perennial Plants 128

4.0 Conclusion 128

5.0 Summary 128

6.0 Tutor MarkedAssignment 128

7.0 Further Reading and other Resources 128

124

Important Definitions - Herbs, Shrubs, Trees, Emphemeral, Annual, Biennial, Perennial 125

The level of development and advancement varies within the plant kingdom. Some plants within the kingdom have more advanced roots, stem and leaves than other; some complete their life cycle within one year, some in two years and some in more than two years. The vascular tissue also varies in terms of arrangement, structure and functions. Some plants have soft stem while others have woody stem.

In this unit, we shall learn about the various important definitions that are commonly used in the plant kingdom.

By the end of this unit, you should be able to:

i. Explain what herbs, shrubs, trees, emphemerals, annual, biennial and perennial trees are;

ii. Give examples of each

These are plants that do not produce permanent shoot systems but die at the end of one growing season.

They do not grow tall and their shoots are soft and greenish and have very little or no tough woody tissue.

There is little growth in diameter and the plants are usually short-lived. The outer surface consists of a thin epidermis in which stomata are present. The green colour of the stem is caused by the presence of chlorophyll and indicates the food manufacturing ability of such sterns. The supports of the leaves depends upon collenchyma, sclerenchyma, and the turgid condition of individual cells. Examples of herbs include sunflower, beans, pepper, tomato etc. Herbs may be annuals or perennials. Herbaceous stems are divided into monocotyledonous and dicotyledonous.

3.1 Herbaceous Monocotyledonous Stem

The vascular tissue of herbaceous monocotyledon stem exists as scattereed bundle of xylem and phloem, although the bundles may be more numerous at the periphery of the stem. Except for the palm trees and certain other monocots, which have an anomalous cambium, no cambium, and thus no secondary tissue is present; the little growth in diameter is dependent upon enlargement of the cells of primary tissues. Even though no definite arrangement of vascular bundles exists - the xylem and phloem never form continuous cylinders of tissues, the xylem is always located on the inner side of a bundle and the phloem to the outer side.

The greater part of the stem consists of parenchyma tissue. The sclerenchyma and collenchyma cells surround the vascular bundles and give strength and support the stem.

3.2 Herbaceous Dicotyledonous Stem

In these stems (fig. 4.1b) the vascular tissue is also arranged in discrete bundles, but the bundles themselves are arranged in an orderly ring and not scattered. The cambium, which is visible between the xylem and phloem, may be restricted to the individual bundles or may be continuous from bundle to bundle.

Whatever the arrangement, secondary tissues are poorly developed and the stem remain non woody.

Frequently the cortex may contain many collenchyma cells and the vascular tissue may contain fibers, both of

which aid in the support of these stems. The parenchyma cells, between vascular bundles, are continuous

with those of the pith and cortex.

Figure 4.2 ^phloem

xylem pith

cambium cortex cork

Fig. 4.1 b

xylem Cambium Phloem Fig. 4.1 a

(a)

a) Herbaceous monocotyledonous stem b) Herbaceous dicotyledonous stem

Briefly explain what herbs are.

3.3 Shrubs

These have hard woody shoot system and grow larger than herbs and do not die at the end of the season.

Their stems form many branches which grow close to the ground. The outer surfaces of the older stems is rough and covered with cork e.g. the bark of trees and shrubs. In this rough surface are lenticels which are really openings beneath which the cells are loosely arranged with many intercellular spaces. Gaseous exchange can take place through these openings. A young woody stem may contain chlorophyll and carry on photosynthesis for a short period, but as the diameter increases and the cork form, this ability is lost.

Infact, young stems are all herbaceous in appearance at first, and the woody characteristics develop as the stems become older. The increase in diameter of such stems results mainly from the production of wood and cork. Examples includeHibiscus, Tecoma, Oleander, ,Pride of Barbados.

3.3.1 Woody Dicotyledonous Stems of Shrubs

In woody dicotyledonous stem, the young stem has vascular bundles, the conducting of mature woody

stems are in the form of concentric cylinders, in which the great development of secondary xylem results in

the characteristic woody condition. The stems usually have much less pith than the herbaceous dicotyledonous

stems. see fig. 4.2