Shoot System, Biology tutorial

Shoot System - Stems, leaves, flower and fruits

Introduction to Shoot System:

Shoot system is the portion of the plant that grows over the soil apart from the modified aerials roots. Shoot system can as well be stated as that part of the plant that builds up from the plumule of the embryo, generally appears over ground (at least at some phase in growth) and bears the buds, leaves, flowers and maybe other appendages. There appears to be no other way to differentiate morphology among a root system and a shoot system apart from by reference to appendages like leaves and flowers that are never borne by roots.

The shoot system comprises of two main organs: (a) Stem and (b) the leaves. For the period of the reproductive phase, flowers and fruits might be seen on different branches of the stem.

Stems:

The stem is the uphill part of the axis of the plant, growing directly from the plumule and bears leaves, branches and flowers. When young, it is generally green in color. A stem comprises of nodes and internodes alternating throughout its length. This is more or less cylindrical in appearance and ends in the terminal bud. Nodes are vaguely enlarged part of the stem from which leaves and buds occur. The buds arise on the axis of leaves and at times build up into branches and flowers. An internode is the part among the two successive nodes. The terminal bud comprises of actively dividing cells (that is, meristematic cells). The entire young tissues of the shoot system grow from these meristematic cells. Newly formed leaves can be observed covering the terminal buds.

  • Stems are erect, procumbent, decumbent, prostrate or creeping.
  • Stem can be smooth, hairy and rough.
  • They can bear roots or not.

Types of stem:

The stem might be (a) aerial (erect, stiff, strong and upright as in herbs, shrubs and trees) (b) sub aerial (weak, not capable to stay upright and trail on ground as creepers or climb up as climbers) or (c) underground (buried in soil and make aerial branches beneath favorable conditions).

Modifications of Stem:

Stems are diversely modified into underground, sub-aerial and aerial stems for performing functions such as manufacturing and storing food, perennation (overcoming adverse climatic conditions), giving mechanical support and protection and for propagating in  a vegetative manner.

a) Underground modified stems: They might appear like roots however you can identify them as stem due to the existence of:

(i) Nodes & internodes, (ii) scaly non green leaves and (iii) buds.

Example: Rhizome, Corm, Bulb and Tuber.

b) Sub aerial modifications of stem: Stems are feeble, thus lie prostrate on the ground or might get partly buried in the top soil. The plants bearing these stems are termed as creepers. Their stems serve up the function of the vegetative propagation.

Example: Runner, Stolon, Offset and Sucker.

c) Aerial stem modifications: The entire stem or its part gets transformed to perform definite functions.

Example:  Stem tendrils, Thorns, Phylloclade and Cladode

Functions of Stems:

A) They grip the leaves in the best place for light which is employed in the process of photosynthesis.

B) Young parts of the stems have chlorophyll; therefore they take part in photosynthesis.

C) They carry out water and mineral salts from the roots to the leaves and prepared food from the leaves to the roots, growing areas and storage organs of the plant.

D) They grip the flowers and fruits in the finest position for pollination and dispersal correspondingly.

E) Some of the stems are modified of special functions like climbing, food storage, protection, water storage and vegetable reproduction.

Leaves:

Leaf is a flattened and extended lateral appendage of stem or branch mounting from its node. It begins from leaf primordium made by the shoot meristem and bears a bud in its axil termed as axillary bud. It is the place of very significant physiological methods such as photosynthesis, respiration and transpiration. Besides protecting the axillary buds, leaf can get transformed into structures for storing the food and water, climbing, vegetative propagation and so on.

Forms of Leaves:

There are mainly two forms of leaves. These are simple and compound leaves.

1) Simple Leaves: A leaf is stated to be simple when it comprises of merely one leaf-blade. The leaf-blade might though become deeply lobed; however this doesn't extend to the mid-rib or petiole. At times the mid-rib of a deeply lobed simple leaf could be splitted at the base where it joins the petiole, to make a simple palmate leaf as in the pawpaw and cassava.

2) Compound leaves: A compound leaf is a leaf splitted into separate laminated units termed as leafless. A leafless has its own leaf base and is either joined to a mid-rib or directly to the petiole head. Leaflets encompass no axillary buds.

Functions of Leaf:

Leaf performs the given functions:

a) Photosynthesis: Leaves prepare their own food in the presence of sunlight.

b) Exchange of gases: Stomata assist in exchange of gases which are significant for respiration and photosynthesis.

c) Transpiration: Evaporation of surplus of water in vapor form occurs via stomata which assist in ascent of sap and cooling of the surface of leaf.

d) Guttation: Exudation of surplus of water having salts occurs in liquid form from leaf margins in plants growing in the humid climate.

e) Modifications for particular functions: In some plants leaves perform functions such as manufacturing and storing food, giving support and protection, vegetative propagation and catching of insects.

Flowers:

A flower is that portion of shoot modified for the sexual reproduction. Flowers are generally generated in groups on a special branch of the shoot termed as inflorescence. Certain flowers however take place singly on the axil of a leaf on the stem. These flowers are stated to be solitary.

A flower comprises of four major parts termed as floral leaves. Such floral leaves are arranged on a vessel in form of rings termed as whorls. The flower is fixed to the stem branch through a stalk termed as the pedicel.  There are some flowers such as Aspilia and sunflower which do not have pedicel. These flowers are stated to be sessile.

The four floral leaves comprise of:

1) Sepals or Calyx

2) Petals or Corolla

3) Stamens or Androecium

4) Carpels or Gynoecium or Pistil

Calyx:

The calyx comprises of all the sepals that make up the outermost whorl. Sepals are generally greenish in color and differ in number and arrangement. In most of the Dicots, sepals range from 4 to 5 in number. In monocots though, they take place in multiples of three, state 3, 6, or 9.

a) The main function of calyx is to protect the inner floral leaves when the flower is still in the bud phase.

b) As sepals are greenish in color, they have chlorophylls and can thus carry out photosynthesis.

Corolla:

The petals of a flower form the corolla that is the next inner whorl to the calyx. Petals are the most conspicuous portion of a flower. They are often brightly colored and scented, particularly in such flowers pollinated by birds and insects. Petals as well differ in number in various plants.

a) Brightly colored and scented petals fascinate the animal pollinating agents like birds and insects.

b) Petals as well offer protection to the stamens.

Androecium:

It forms the third whorl of the flower and is build up of one or more stamens. Each and every stamen comprise of filament, anther and connective.

Gynoecium:

This is the fourth and second necessary whorl of the flower. Gynoecium is a female portion of the flower including the inner whorl of megasporophylls in the form of carpels bearing the ovules. It comprises of ovary, style and stigma.

Seeds and Fruits:

Seeds:

A seed is a structure which generates from the ovule after fertilization. A seed comprises of an embryo and one or two cotyledons. It might have an endosperm or food reserve. The embryo is build up of a radicle or future root system and a plumule or future shoot system. The seed is thus a potential young plant awaiting certain favorable situations for germination.

Fruits:

A fruit is the structure that generates from the ovary after fertilization. Instantly after fertilization the ovary starts to rise in size as an outcome of accumulation of food substances. This is followed through gradual changes in color. The wall of fruit develops into a pericarp or fruits coat. At maturity the pericarp is either solid and dry or soft and fleshy. A fruit can thus be regarded as a mature ovary, having one or more seeds.

Categorization of fruits:

There are numerous criteria used in the categorization of fruits, however the most popular is according to the nature of the ovary from which the fruit develops. By using this principle, fruits can be classified into three main groups. These are:

A) Simple fruits:  These fruits are formed from a single flower in which the gynoecium is formed either a single carpel or several united carpel.

B) Aggregate fruits:  These are fruits produced from a single flower in which the gynoecium comprises of lots of free carpels.

C) Composite or Multiple fruits:  These fruits are formed from lots of flowers, whose ovaries unite altogether, with other portions of the flower after fertilization.

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