Monocot and dicot Stems

Introduction

Stems are classified basing the way the vascular tissues are scattered. The vascular bundles are irregularly scattered in the monocot stem, additionally,  there is a poor distinction between the cortex and pith. In a monocot stem, the peripheral layers contain smaller bundles and they become thicker and larger towards its centre. In some of the plants, the vascular bundles are formed out of one or more indistinct rings called the band of sclerenchyma. Monocot is identified by a V-shaped arrangement of xylem cells. Dicots on the other hand have multicellular epidermal hairs all over the epidermis. They also have chollenchymatous hypodermis; pith; differentiated ground tissues; and a limited number of vascular bundles in a concentric arrangement. The table below shows the major differences between monocot and the dicot stems.

 

 

 

 

 

Criteria

Dicot Stem

Monocot Stem

Arrangement  of Vascular Bundle

In dicot stems, the vascular bundles are arranged in the form of one or two broken rings.

In monocot stems, the vascular bundles are scattered across the stem with no definite arrangement.

Bundle Sheath

Absent in the dicot stems

The bundle sheath surrounds the scattered vascular bundles of a monocot stem.

Hypodermis

The hypodermis of the dicot stem is made up of the collenchyma.

The hypodermis in the monocot stem is made up of the sclerenchyma.

Presence or absence of Cortex  and  Stele

The vascular system in dicots contains 2 distinct regions, I,e, cortex, and stele.

The vascular bundles are scattered, and they lack the distinct cortex and stele.

 

 

Growth in a Stem

The growth of the stem is divided into, primary and the secondary growth. Primary growth occurs at the apical tips of the stem as a result of rapidly dividing merismatic tissues found in the stem. Secondary growth results in the thickening of the stem due to the extension of lateral meristems and it is absent in the herbaceous plants as they lack cambium which is primarily responsible for it. The primary growth enhances the linear growth of plants but the secondary growth accelerates the thickness of the bark of the stems.

 

Classification of Stems

Apart from the secondary growth, stems can also be classified based on the location where they grow . They are Underground stem, Aerial stem, and Subaerial stem 

 

Underground stems

The underground stems remain beneath the topsoil but they produce aerial shoots that reach well above the level of the soil. Roots of an underground plant is typically  superficial  and they help in  in the storage of food and perennation as seen in the onions and potatoes. They are also capable of vegetative propagation; a type of asexual reproduction . Underground stems can also be classified as rhizomes, tubers, bulbs, and corms. A rhizome has a thick underground stem with distinct nodes and internodes ( Ginger). The tuber has an horizontal underground stem with their tips help in the accumulation of stored food (Potato). Bulb is a short underground stem present with a fleshy base and leafy scales. 

 

 Subaerial Stems

Subaerial Stems run parallel to the ground. They have nodes that transform into roots at some point. Sub-aerial stems are classified as a runner, offset, stolon, and the sucker. Runner grows parallel to the ground with a creeping stem. The nodes on the lower side of the stem give out adventitious roots at regular intervals. A runner develops from the axils found on the lower leaves of the aerial stem. Offset stem s shorter and thicker than the runner commonly found in aquatic plants. Stolon is similar to a runner that arises from the lower part of the main axis. Sucker stems are very similar to the stolon but they grow obliquely upwards to develop into a new plant. 

 

Aerial Stems

Aerial Stems are formed above the ground level. They are classified into thorns, tendril, phylloclade, cladode, and bulbil. Thorns are the modifications of a typical stem that looks hard, woody and sharp outgrowths (roses). Tendril has a slender, wavy, twining strands help in supporting the plant`s weight by encircling around tall trees. Phylloclade is a green, flattened or cylindrical stem looks like a leaf capable of conducting photosynthesis. Cladode is a modification of the phylloclade but they have additional internodes. Bulbil stems are actually the modified axillary buds. They look like fleshy and rounded as they store food in them.

 

Questions 

  1. Elaborate  the fundamental differences between monocot and the dicot stems.

  2. Explain the mode of arrangement of Vascular Bundles in monocot and the dicot stems.

  3. Mention the cell types present found in the monocot and dicot stems.

  4. Classify stems based ontheir location and explain about aerial stems.

  5. What are subaerial stems? Briefly explain its subtypes.

  6. Distinguish between the primary and the secondary growth. If a tree is growing tall but not in its breadth, then what kind of growth is more active?

 



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