Topic3: How plants grow

      The plant will continue to grow upward and outward as its cells multiply. New leaves will appear, as will flowers in many plants. As the plant grows, it will continue to need the proper nutrients from the soil and water as well as sunlight or the right artificial light. Plants in good health will eventually reach their full height and maturity, which is dependent upon their specific variety. Once a plant reaches maturity, it will reproduce. Plants can reproduce as long as they have a male and a female reproductive system. This can happen in plants that are hermaphrodites or with a separate male and female plant that are near each other. Plants also reproduce, or are propagated, in the following ways:

·         Splicing two plants together- such as a Red Delicious apple with a rare apple variety

·         Runners or Stolons- as seen on the strawberry plant

·         Adventitious buds - such as those seen on the trunks of trees that have been cut down

·         Suckers- as seen on Elm trees, tomatoes and roses

·         Bulbs - plants like the onion, garlic and tulips reproduce by forming new bulbs

·         Corms - glads and crocuses reproduce by forming new corms

·         Tubers - similar to bulbs, the dahlia and potato reproduce more tubers

      First, let’s look at primary growth. Primary growth extends the length of a plant both aboveground and belowground. Since humans generally live aboveground, we usually only see the aboveground parts of a plant: the shoot system. The entire shoot system, no matter how large or small, owes its beginnings to a small region of the plant called the shoot apical meristem. An apical meristem is a region of high cell division that contributes to the extension of the plant. The shoot apical meristem is an apical meristem that is in the shoot system, as opposed to the root apical meristem that is, you guessed it, in the roots. It is only through the activity of the shoot apical meristem that the plant grows taller. The shoot apical meristem is found at the tip of the plant stem, so growth extends upward from the top of the stem, not the bottom. Those bottom leaves aren’t going anywhere until they fall off the plant. That means if you carve your name into the trunk of a tree, it will still be there many years later. One more meristem is the intercalary meristem. This is a region of rapid cell division at the base of nodes. This type of meristem is only found in monocots, so don't go looking for it on eudicots. You’ll be looking a long time. These are particularly important to monocots because they allow stems to elongate quickly and also for leaves to regrow quickly if they have been damaged. Just like a human body has all its different parts, a plant body has parts that are the same on every plant, though they may look different in different species.

The parts of a shoot system are the:

·         Stem (nodes + internodes) 

·         nodes are where leaves attach to the stem

·         internodes are the spaces on the stem in between the leaves

·         Leaf (petiole + blade)

·         Branches, which grow out of axillary buds

·         Reproductive parts (the flowers and fruit)

     A leaf is made up of a blade and a petiole. The blade is the flat green part that you usually think of as the leaf, and the petiole is just the little stem that attaches the blade to the main stem. In between the leaf primordia, where new leaves form, and the stem below, are the axillary buds. These will form branches, which will have their own apical meristems on the ends. Axillary buds are often protected by bud scales. A bud scale is a modified leaf that covers the delicate bud until it starts to grow into a shoot. Most of the parts named above are visible as they originate on the shoot apical meristem. The shoot apical meristem is comprised of leaf primordia, which turn into leaves, and the apical dome, where the stem elongates. Under a microscope, the tip of a plant shoot looks like this:

     Now we know how a plant gets taller and its roots get longer. Even a big tree with an enormous trunk starts out as a puny seedling. Popeye eats a lot of spinach to grow big and strong, but what do spinach plants eat? The width of a plant, or its girth, is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity. However, the cells they make grow outward rather than upward or downward. Eudicots use lateral meristems to add to their width; monocots, however, do not experience secondary growth. We’ll come back to them later. The lateral meristems that produce secondary growth are called cambiums, which just means a tissue layer that adds to plant growth. The two important ones for secondary growth are the vascular cambium and the cork cambium. The vascular cambium produces more vascular tissue, which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem. Here is what that looks like: