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Stems
Stems provide a structure to support the plants leaves, flowers and fruits. Stems also provide the means by which water and nutrients are transported around the plant. Water and nutrients are transported from the roots through the stem to the rest of the plant. A sugar solution is transported from the leaves through the stem to the rest of the plant as needed. Stems are incredibly complicated structures. The growing tip, known as the terminal bud, is where cells constantly divide to elongate the stem. Leaves or branches will develop from the terminal bud as it continues its elongation. The stem ensures leaves are held in the best possible position to receive the maximum amount of light. Look down on a plant from the top and notice how most of all the leaves can be seen. Stems also ensure flowers are positioned best for pollinators such as bees or positioned best for wind pollination. Herbaceous stems, which are soft and flexible, are made up of six clearly defined areas:
The cuticle reduces evaporation and is a layer of added protection against mold invasion The phloem vessels carry water up from the roots to the rest of the plant. The xylem vessels carry sugars and minerals downward through the plant. On some plants these stem hairs exude a sticky liquid which traps insects. A perfect insect barrier! Woody stems in contrast to herbaceous stems are harder and less flexible. Just look at a herbaceous stem growing beside a tree trunk and notice the vast difference, But, at the trees seedling stage it was also just a herbaceous stem. And consider the fact that a trees branch tip are always herbaceous, green and soft textured. So, what makes a tree trunk so different from a soft and flexible herbaceous stem? Well, in stems that become woody a layer, which is known as the vascular cambium, separates the xylem and phloem cells. The cells of the vascular cambium divide in three directions. Cells that grow on the inner side of the vascular cambium layer develop thick and lignified walls which die off. They then become water conducting cells of the secondary xylem, or better known as wood cells. Cells growing in a sideways direction add cells to the meristem as its circumference around the growing wood core increases. Vascular cambium cells growing outward form the secondary phloem. Some of the secondary phloem cells have thin walls and conduct food. Others have thick walls which gives a physical support to the thinner food conducting cells. There is also a layer known as the cork cambium which becomes active after the stem has begun to thicken. The cork cambium is responsible for the formation of the outer tree's bark and replaces the epidermis. This cork layer is several layers thick and gives the tree added protection and with a natural substance called suberin it is also waterproof. The cork layer also allows gases, oxygen, carbon dioxide, to exchange between the living tissues of the bark and the atmosphere. If you want any further information on stems please feel free to
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