The Stem

Introduction

You have learnt about the root as the part of the flowering plant that is below the soil. In this unit you will learn about the stem. The stem is the portion that grows above the soil. The stem develops from the plumule and bears leaves, branches and flowers. A young stem is green in colour. The stem has nodes and internodes. Leaves and branches normally develop from the nodes.

 Forms of Stem

Here, we are going to look at the structures different plants possess as stem. There are two major forms the stem normally takes - they are aerial (those stems above the ground) or underground (as the name implies, they are developed below the ground). Aerial stems are usually erect or upright and strong while some are weak and frail on the ground or support themselves by climbing on stronger and erect objects.
Examples of plants with erect stems are mango, cashew, etc. Those with weak stem are pumpkin, dodder, etc. Can you think of other examples? While some stems are above the ground, some are permanently underground and sometimes produce aerial shoots. Examples include onion, ginger, etc. Let us now examine the stem closely.



The stem that is erect, cylindrical and has no branches is called caudex. Such stems have the mark of scars of fallen leaves on the body. Example, palms. Stems with hollowed internodes and joined nodes are called culm, e.g. bamboo. As earlier pointed out, there are some underground stem that produce erect unbranched shoots which late bear single or group of flowers. Such shoots are called scape.

Weak Stems:

Weak stems can be classified as (i) Trailers, (ii) creepers, (iii) climbers. Trailers have their stem running on the ground without rooting at the nodes. When the stem lies prostrate on the ground it is called procumbent while after trailing for some distance, it rises at the apex, it is called decumbent. If the stem has many branches and trail at direct directions, it is said to be diffuse. What distinguishes trailers from creepers is that creepers produce root at the node while trailers may not. A plant is said to be a climber if its weak stem climb other objects or plants around it.

Nodes and Internodes:

The node is the point on the stem or branch where one or more leaves develop. While the space or distance between two nodes is termed internodes.

The Bud:

A bud is a young undeveloped shoot consisting of a very short stem and tender leaves. The bud can be at the apex of the stem (usually called terminal or apical bud) or the branch or axil of the leaf (axillary bud). In some cases, buds develop beside the axillary bud. Such buds are called accessory buds. Buds can develop on other parts of the plant such as the leaf (foliar buds), root (radical buds). When buds arise on other parts of the stem or branch outside those parts mentioned above, they are called adventitious buds.

 Natural ways by which buds are protected:

The buds are very important because they produce the stem, branches and leaves. To protect it from damage they are naturally protected by:

1. Overlapping and fielded leaves 
2. Some are covered with hair or gummy secretion 
3. Dry and scaly outer leaves called bud scales 
4. Coated by wax on the leaf surface control water loss and getting wet. 

Modif ication of Buds:

Apart from the bud developing into stem, branch or leaf, some buds develop into tendrils (rope-like structure), as in fluted pumpkin. Some others become thorns while some become modified into special reproductive bodies called bulbils. Readers are encouraged to observe these features out in a field for understanding of these features. better

Habits of Plants

The habits of plants are known from the mode of their life, the height of the plants, the nature of the stem and the duration of life. Herbs: Plants that are small and have soft stems are called herbs.
Shrubs: Plants in this group are of medium size but with hard and woody stem. They do not have distinct trunk but branch a lot near the ground. Such plants are not as small as herbs and are not as large as trees. Trees: Trees consist of very large plants with hard and word trunk and branches.
Climbers: Plants classified as climbers have tiny and long stems with or without branches. They are very weak and so cannot stand erect on their own.
By the duration of life, plants can be describe as:

1.  annual plants (those plants that when planted can reach their maturity and are harvested the same year) biennial (those that live for two years) perennials (those ones live for many years, normally from three years and above). 

Modif ications of Stems

We have identified that while some stems are aerial, some are underground. Those stems whether aerial or underground take different shapes and sizes. Different functions are performed by the stem, some of which include:

1. helping the plant to survive throughout the year in bad or adverse seasons 
2.  vegetative propagation by which the stem of plant is used to produce new ones 
3. some store food for the plant 
4.  some stems that take the form of tendrils allow the plant to attach itself to other plant around it. Let us consider the modification under two headings 

namely:

1. Underground modification of stems: Underground stems are usually thick and fleshy with enough food stored in it. Plants with such stem normally shade their leaves during unfavourable condition or at maturity and send out aerial shoots during favourable conditions. They can be distinguished from roots with the presence of nodes and internodes, scale leaves and buds on their body. However, because they are underground, they develop root like features, non-green colour. Modified underground stems can be classified into rhizome, tuber, bulb and corm. 
2. Rhizomes possess the characteristics of prostrate, thick stem that grows horizontally but sometimes vertically in the soil. They have clearly displayed node with short or long internodes bearing scaly leaves at the nodes. The buds are found both at the leaf axil and apex (terminal bud). Some rhizomes have branches developing from the axillary buds while some are unbranched. They produce aerial shoots during vegetative season. Examples of rhizomes are ginger, water- lilly, ferns, etc. 
3. Tubers are swollen ends of underground branch arising from the axillary bud. Found on the body of tubers are `eyes' or buds, which can develop into new plant. The swollen nature of tubers is due to the food deposit. Examples of tubers are yam, potato, cassava, etc. 
4.  Bulb is another form of modified underground stem. These are usually fresh scaly leaves or foliage often regarded as scales growing from the stem and around it. Adventitious roots are always developing from the base during vegetative growth. While the fleshy leaves store food, the dry ones give protection. Common examples of bulb are onion, lillies, garlic, etc. 
5. Corm is much like rhizome except it is of large fleshy stem, more or less rounded in shape. The heavy deposit of food materials makes it larger in size. Few buds are found at the axil of the scale leaves. Examples include coco-yam, saffron, etc. 

Aerial Modif ications

Aerial stems are modified into runner, stolon, offset, tendril, thorn and phylloclade to perform different functions.

1. Runner: The stem of some plants are structure in tiny slender form that creep on the ground. Normally characterised by short or long internodes (depending on the plant) the axillary buds give rise to new runner. The development of roots at certain intervals enables the new runner to grow independent of the mother plant in case it breaks off. 
2. Stolon: The stolon has the characteristics of the runner. However, stolon starts by growing upward and later bend to the ground. Where it touches the soil, it develops roots and bud and rise again and continue to grow laterally (sideways). Each of these buds can grow as a new plant, e.g. wild strawberry. 
3. Offset: This is another form of stem. It is a horizontal branch from the mother plant characterized by thick cluster of leaves at the apex with roots at the based. Like the runner, sudden break from the mother plant gives rise to a new plant. Examples of offset are water-lettuce and water hyacinth. 
4. Tendril: This is a tiny rope- like structure used by the plant to climb other objects. The tendril may develop from the axillary bud or terminal bud. The climbing is made possible as the tendril curled round any near by object.
5. Thorn: This is another form which stems of plant can take. These are hard pointed structures arising from the body of plant. Thorn is said to be modified stem because they develop from the position when the normal branch takes off. 
6. Phylloctade: This type of stem modified into thick flat or cylindrical structure weakly joined at successive points. The leaves are modified into spines, e.g. prickly pear, cocoloba, christmas cactus. 

 Summary

Stems are generally considered to be part of the flowering plant above the ground. But there are exceptions, some stems are below the ground and are said to be underground. There are erect and strong stems and weak stems. The weak stems  are said to be weak because they need the help of something, stone, another plant or whatever to spread their leaves for photosynthesis. Among the stems referred to as weak stem, there are climbers of various sorts and creepers. Some plants have modified their stems