Shoot system of the plant is an outgrowth originates from the plumule of the seed’s embryo, above the ground. A term shoot is generally interchangeable with the stem, as it constitutes the major part of the shoot system. The morphology and physiology of the shoot system are more complex than the root system of the plant. The main stem is the vertical axis that comprises of two segments, namely nodes and internodes.
Nodes are the segments, where the leaves get fixed, and internodes are the segments in the middle of two nodes. A shoot is a highly branched and complex structure of the plant that anchorage leaves, buds, flowers and fruits.
Content: Shoot System of Plant
Definition of Shoot System
A shoot itself refers to the main stem of the plant, but a shoot system can define as the complex network of various structures like branches, leaves, buds, flowers and fruits attached to the main stem. A shoot or a shoot system always grow upwards to the ground and performs multiple functions like photosynthesis, storage, reproduction, transport, hormone production etc.
Characteristics of Shoot System
A shoot or seedling originates from the plumule of the seed’s embryo and shares the following morphological features:
It functions as a skeleton by constituting a major part of the shoot system and firmly supports the other components like leaves, buds, flowers and fruits. The main stem originates through the direct prolongation of tigellum of the embryo and gives rise to the lateral stems, comprising leafy appendages, buds etc.
These are the flattened structures that hit the node of the main stem and the region in the middle of two nodes, refers as internode. The angle that forms between the leaf at the node section and the vertical stem commonly refers to “Leaf axil”. These emerge out by the differentiation of the shoot apical meristem and arrange either in a basipetal or acropetal pattern. A leaf mainly comprises of three elements, namely:
- Leaf base: It fixes the attachment of leaf to the stem’s node.
- Petiole: It is a stalk-like appendage that joins a leaf base to the leaf lamina.
- Leaf lamina: It is the leaf blade that comprises of midrib, veins and veinlets.
It commonly refers as a lateral bud or lateral meristem that is found attached to the leaf axil. An axillary bud is of two types:
- Type-I or Vegetative kind: It promotes the growth of the vegetative branch.
- Type-II or Floral kind: It gives rise to the flowers from the rudimentary reproductive tissues.
It commonly refers as a terminal bud or terminal meristem that is found at the shoot apex of the plant. These are small, compact, composed of apical meristematic tissues and surrounded by leaf primordia. Apical bud includes three meristematic layers of cells, namely Protoderm, procambium and ground meristem. The ground tissue of apical meristem further divides and differentiates to form vascular tissues for the conduction of food materials.
It constitutes the reproductive part of the shoot system of the plants that belong to the members of Angiosperms that are meant to reproduce sexually. A flower involves four characteristic whorls, namely:
- Calyx: The arrangement of sepals collectively refers as calyx that appears green-coloured and is a leaf-like structure present towards the flower’s base.
- Corolla: The arrangement of petals collectively refers as corolla that is present above the calyx and is bright-coloured.
- Androecium: Stamen that consists of filament and anther will collectively refer as androecium that will constitute the male reproductive part.
- Gynoecium: Carpel that consists of stigma, style and ovary will collectively refer as gynoecium that will constitute the female reproductive part.
Based on the function of floral parts, a flower can classify into the following two types:
- Accessory organs: It includes calyx and corolla that attract the pollinators like a honey bee, butterflies etc.
- Reproductive organs: It includes androecium and gynoecium that encourages the growth and fertilization of the flower.
It is the reproductive structure that indicates the maturity or the age of the plant. Plants those lack seed or the reproductive parts, will produce fruits asexually and are known as “Parthenocarpic fruits”. A fruit generally includes pericarp and seed formed after complete fertilization of an ovary and ovule respectively. When a pericarp is thick, it differentiates into three distinct layers, namely outer epicarp, middle mesocarp, inner endocarp.
Shoot System Functions
A shoot system of the plant body performs specific functions like:
Protection: In some plants, a stem comprises of hairy or spiny structures on its surface that harbours a plant from the predators. Some plants like bracken produce toxic materials that also keeps away the grazing animals.
Support: A Ground tissue like sclerenchyma and collenchyma also provide strength and rigidity to the stem, so that it can stand straight and embrace various components of the shoot system like leaves, lateral branches, buds, flowers etc.
Photosynthesis: Leaves of the shoot system composed of chlorenchyma tissue, contains the high amount of chlorophyll pigment that absorbs light energy to produce sugar, to maintain the power and metabolism of the plant.
Transpiration: Both leaves and stems can undergo transpiration via stomata and lenticels, that allow gaseous exchange between a plant and the surrounding.
Conduction: A main stem of the shoot system participates in the transport of prepared food by the leaves to the other parts via phloem vessels, and facilitates water and mineral conduction trapped by the roots to the other components via xylem vessels.
Hormone production: A shoot tip produces auxin (a growth regulatory hormone) that stimulates the vertical growth or height of the plant and restricts the growth of axillary bud. A cytokinin is also a growth-regulating hormone that can conquer the inhibitory effect of auxin, by stimulating side branching or the growth of axillary bud. The cytokinins increase the diameter or thickness of the plant and give a bushy appearance. Therefore, both the hormones can manipulate the growth pattern of the plant and can use widely in the field of agricultural science.
A shoot develops after the embryogenesis, where a zygote inside an ovule goes through successive mitotic division to form a mature embryo. A mature embryo comprises of five distinct regions that we can understand by the diagram given below:
A top green part will produce seed leaves or cotyledons, in between which a shoot apical meristem is present. The shoot apex or the terminal bud will stimulate elongation of the plant. The region below the apical meristem will encourage the growth of the main stem. The below two layers will give rise to the root system of the plant.