You possibly know about a stem since you were a kid. Then what are we going to do now? We will know more about a stem! Stems play an important role in any plant. No, it is not just transportation of water and minerals! There is more to the story indeed! Let us look at the details of plant stems in this chapter. We will look at their characteristics and types as well. This is an important concept and will help you score better in your exams!
What is a Stem?
The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. It develops from the plumule of the embryo of a germinating seed. It bears nodes and internodes. What are nodes and internodes? The regions of the stem where leaves are born are called nodes. On the other hand, internodes are the portions between two nodes.
The stem bears buds, which may be terminal or axillary. Now, let us look at what the functions of a stem are.
Functions of the Stem
The stem performs the two major functions of
- Spreading out branches bearing leaves, flowers and fruits.
- Conducting water, minerals and photosynthates.
However, many a time stems also have various modifications. These help the stem to perform the function of storage of food, support, protection and vegetative propagation. Before we proceed to know more about such modifications, let us look at the various characteristics of stems.
Characteristics of Stems
- The stem is an ascending axis of the plant and develops from the plumule and epicotyl of the embryo.
- It is generally erect and grows away from the soil towards the light. Therefore, it is negatively geotropic and positively phototropic.
- The growing apex of stem bears a terminal bud for growth in length.
- In flowering plants, the stem is differentiated into nodes and internodes.
- The lateral organs of the stem (i.e., leaves and branches) are exogenous in origin (i.e., from the cortical region).
- The young stem is green and photosynthetic.
- Hair, if present, are generally multicellular.
- In mature plants, stem and its branches bear flowers and fruits.
Now that we know quite a few things about stems, let us look at the various modifications of stems. This is an important aspect of this chapter. Hence, you must pay extra attention to it.
Modifications of Stem
Too often, stems are modified to perform a range of functions. Below, we will discuss in brief about the various modifications of stems.
- Modification of Stems for the Storage of Food: Underground stems of potato, ginger, turmeric, zaminkand, colocasia are modified to store food in them. They also act as organs of perennation to tide over conditions unfavourable for growth.
- Modifications for support: Stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb such as in gourds (cucumber, pumpkins, watermelon) and grapevines.
- Stem modifications for defence: Axillary buds of stems may also get modified into woody, straight and pointed thorns. Thorns are found in many plants such as Citrus, Bougainvillea. They protect plants from browsing animals.
- Modification of stems for photosynthesis: Some plants of arid regions modify their stems into flattened (Opuntia), or fleshy cylindrical (Euphorbia) structures. They contain chlorophyll and carry out photosynthesis. Besides these, stems can also be modified for vegetative propagation.
Thus, we see the characteristics and functions of the stem. We also went through the concept of various modifications of stems. A modification is important in various organs of the plants as they help to perform separate functions. At times, these functions are crucial for the existence of the plants.
Solved Examples for You
Q1: Explain the modification in stems for vegetative propagation.
Ans: At times, the stems of the plants can be modified to serve various purposes. They include:
- Runner: Underground stems of some plants spread to new niches and when older parts die new plants are formed. e.g., grass and strawberry
- Stolon: In these plants, a slender lateral branch arises from the base of the main axis and after growing aerially for some time arch downwards to touch the ground. e.g., mint and jasmine
- Offset: A lateral branch with short internodes and each node bearing a rosette of leaves and a tuft of roots are found in aquatic plants. e.g., Pistia and Eichhornia.
- Sucker: In these, the lateral branches originate from the basal and underground portion of the main stem, grow horizontally beneath the soil and then come out obliquely upward giving rise to leafy shoots. e.g., banana, pineapple and Chrysanthemum.
Q2: Ginger is a stem which can be differentiated from root because it:
- Grows parallel to the ground
- Stores food
- Lacks chlorophyll
- Has nodes and internodes
Ans: A rhizome is a thick horizontally growing stem. It has nodes and internodes as well as scale leaves, axillary buds, adventitious roots and a terminal bud. Hence the answer is d.