The Stem
The vascular bundle in the stem of a plant differs in the type of plant. A monocots vascular tissue is randomly distributed throughout the stem with no particular pattern whereas a dicot has gathered and orderly rings throughout the stem. Horizontal above-ground stems are called stolons or runners. These produce new plants where nodes in the stem touch the ground. An example of this is a strawberry plant. The vascular bundle in vertical stems can be modified. For example stems of cacti are modified to store water and tendrils of grape plants are modified to wrap around a supporting structure. Horizontal under-ground stems are called rhizomes. These are either long and thin such as grass or thick and fleshy such as iris plants. Rhizomes are able to reproduce asexually because each node of the stem bears a bud. Some rhizomes have enlarged sections called tubers such as potatoes. Some stems are bulbous underground called corms which are covered in papery leaves and lie dormant underground in the winter.
Function of the Stem
Like the root, stems have a supporting and nutritional role. The support that the stem provides is holding the plant up towards the light. The nutrient transport role has three functions:
1) Transport water and dissolved mineral nutrients from roots to leaves through the xylem
2) Transport newly manufactured food molecules from leaves to roots for storage through the phloem
3) Transport food stored in roots to any part of the plant through the phloem
1) Transport water and dissolved mineral nutrients from roots to leaves through the xylem
2) Transport newly manufactured food molecules from leaves to roots for storage through the phloem
3) Transport food stored in roots to any part of the plant through the phloem
Stem: Epidermis
The epidermis protects the inner tissue of the plant. There is a covering over the epidermis called the cuticle. The cuticle and the epidermis are perforated by the stomata just like the leaf is. These stomata allow for gas exchange in and out of the plant. The epidermis of young stems contains chloroplast and can perform photosynthesis.
Stem: Cortex and Pith
In the cortex of a dicot stem, the cells are irregular in shape and arrangement. The pith is similar to cortex tissue. A high water pressure in the pith and cortex makes the cells fully expanded. A condition called turgor helps to keep young stems upright. The function of the cortex is to store food in the form of starch. The pith stores water and small amounts of food. The pith and the cortex do not play a role in food and water transport.
The Root
The major role of the roots is to act as an anchor in the soil to hold the stem in place. The roots also benefit the biosphere by anchoring the soil which prevents erosion. The fibrous root system is made of a large number of slender roots. A taproot is a plant that grows on large root such as a carrot. A taproot is dominant, fleshy, usually stores food and is typically consumed as vegetables by humans.
Nutrient Transport Functions of Roots
The second major role of the roots is to transport nutrients within the plant. To do this the plant performs three functions:
1) The roots absorb water needed by leaves for photosynthesis and replaces the water that is lost during transpiration
2) The roots absorb dissolved minerals and regulate the quantity of minerals that enters the rest of the plant
3) The roots store starch that was manufactured from food and produces by the leaves. Some of the stored food is used by he cells of the root and some is transported to other parts of the plant that require it.
The absorption functions of the root are performed by the outer surface of the root cells. Root systems are divided and subdivided into networks in order to the surface area. Roots absorb the more when the external surface area is at a maximum.
1) The roots absorb water needed by leaves for photosynthesis and replaces the water that is lost during transpiration
2) The roots absorb dissolved minerals and regulate the quantity of minerals that enters the rest of the plant
3) The roots store starch that was manufactured from food and produces by the leaves. Some of the stored food is used by he cells of the root and some is transported to other parts of the plant that require it.
The absorption functions of the root are performed by the outer surface of the root cells. Root systems are divided and subdivided into networks in order to the surface area. Roots absorb the more when the external surface area is at a maximum.
Root: Epidermis
Root hairs are an extension of close-fitting cells that make up a continuous covering around the root exterior. The root exterior is the epidermis. The function of the root epidermis is to protect and enclose the interior root structures and absorb water and dissolved minerals from soil. The root hairs increase the surface area that is able to absorb materials. The epidermis cells of the root do not contain chloroplast and cannot produce food. Roots must perform cellular respiration to obtain energy and energy means life. Glucose is required for cellular respiration and comes from broken down starch that is stored in the cortex.
Root: Cortex
The cortex of the roots lies just inside of the epidermis. In a dicot plant the cortex takes up most of the volume of the roots. In the cortex there are air spaces created by the irregular shape and arrangement of the cells. Cortex cells contain large vacuoles for food storage.
Root: Endodermis
The endodermis of the root lies just inside the cortex. These cells fit closely together. There are specialized cells in the endodermis that act to endure that materials travelling into the center of the root are carefully filtered before moving on. The endodermis keeps many harmful substances out while still permitting nutrients and water into the roots vascular system. There is only one way into the vascular tissue of a plant and that involves passing through the selectively permeable membrane of the endodermis cells. The endodermis cannot block the diffusion of small molecules such as water and oxygen, however can block the passage for large particles such as dissolved minerals. The endodermis controls what goes into the vascular tissue of the plant and the quantity of it.