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Shoot System: Stem and Its Characteristics, the Shoot Apex, Origin of Lateral Branches and Types of Stem
Characteristics of Stem
- The stem is the ascending portion of the axis of the plant
- Develops directly from the plumule, the stem is provided with nodes and internodes which may not be distinct in all cases
- When the stem or the branch ends in a vegetative bud it continues to grow upwards or sideways, bears leaves, branches and flowers on nodes
- They are negatively geotropic and positively phototropic, and the stem bears different kinds of buds, e. g. , axillary, terminal and floral buds.
Differences between Stem and Root
Stem | Root |
---|---|
The stem is the ascending portion of the axis and it turns towards light | The root is the descending portion of the axis of the plant and grows away from light |
Develop from plumule | Develop from radicle |
Young stem is green colored because of chlorophyll | Non-green because chlorophyll absent |
Divided into nodes and internodes | Not divided into nodes and internodes |
Bears leaves, vegetative and floral buds | Absent |
No cap present at the apex | Root cap is present at the apex |
Positively phototropic and negatively geotropic | Negatively phototropic but positively geotropic |
Origin of lateral branches is exogenous (originating from outer layers) | Origin of lateral roots is endogenous (originating from inner layers) |
The shoot hairs prevent evaporation of water from the surface of the stem | Root hairs absorb water and minerals from the soil |
Branches are exogenous i.e.. they arise from the outer layers | Branches are endogenous i.e.. they arise from the pericycle |
The Shoot Apex
- Shoot apex is the terminal, dome-shaped part of the shoot and is formed of meristem called apical meristem is responsible for the development and differentiation of primary permanent tissue and mainly causes growth in lengths.
- According tunica-corpus theory proposed by Schmidt (1924) consists of an outer layer of tunica: 1 - 3 layer thickness, forms protoderm for epidermal tissues and is responsible for surface growth and inner mass of corpus: is multi-layered zone of cells which in all directions, forms procambium for vascular tissues and ground meristem for ground tissues. These cells also form leaf primordia for a newly developing leaf.
Origin of Lateral Branches
Branches arise from axillary buds present in the axil of leaves. Each axillary bud is small, compact, underdeveloped shoot covered with a large number of overlapping leaf primordia. Internodes of this bud enlarge and develop into a branch. Therefore, the development of branches is exogenous (exo = outside) .
Types of Stem
There are three types of stem: Underground stem, Aerial stem and Sub- aerial stem.
- Underground stem: buried in soil and produce aerial branches under favorable conditions only and storage of food.
- Aerial stem: erect, rigid, strong and upright as in herbs, shrubs and trees.
- Sub-aerial stem: weak, short, lateral branches, unable to stay upright and trail on ground as creepers or climb up as climbers.