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[Screaming Eagle]

Plant Growth and Roots

In our last installment, we discussed the development of plants through evolutionary processes, the general structure of plant bodies and the different kinds of tissues. In this episode, we will explore plant growth patterns as well as the specific structure and function of roots and their processes.
Plants are different from animals. Surprised yet? Let me know when you're surprised. Animals simply enlarge the organs they were born with. They don't add organs as they mature. Plants display what's called open growth. They continue to add new organs (branches, leaves, roots) by enlarging from bottom to top. Plants overcome their stationary disadvantage by growing toward light, water and nutrients and away from harmful situations. Much of the movement that we see in informal upright trees as well as the influences of wind, snow and climate extremes on cascading and windswept trees are a result of this open growth pattern.
This growth is based on the activity of meristems. These are perpetually embryonic cells that are undifferentiated and give rise to new cells and cell types as the need arises from the plant. Meristems give rise to both leafy tissue and stems as well as reproductive flowers and the eggs and sperm they contain. Plants do not establish specific reproductive cells during early embryonic development.
Basically, there are 2 kinds of meristems: apical and lateral. Apical meristems (as the name implies) conduct growth activity in the tips (apex) of roots and stems. These cells facilitate extending growth upward toward light and direct roots to grow downward toward water. This type of growth is called primary growth and the xylem and phloem produced by apical meristems is called primary xylem and primary phloem. (Yes, there is also secondary xylem and secondary phloem).
Lateral meristems are like cylinders of divided cells in the stems and roots that cause these parts to become thicker. This increase in girth is called secondary growth and is different from primary growth in that it is demonstrated by thickening rather than lengthening. Most grasses (monocots) do not have secondary meristematic growth so they stay flexible.
Lateral meristems are also divided into 2 types: vascular cambium and cork cambium. Vascular cambium is a type of lateral meristem that produces secondary phloem and secondary xylem (the transporting vessels for water and nutrients) and also known as wood. Cork cambium generates a waterproof outer part of the bark of trees and shrubs.
Primary growth pushes growth upward as a result of growth from the apical meristems. Trees get taller and bushier as a result of growth from the tips and the roots expand and extend themselves sometimes for miles. Trees also get thicker in girth by adding layers of vascular and cork cambium as a result of secondary growth. Woody plants have secondary growth while herbs and grasses only have primary growth. The vascular cambium produces larger cells in spring and smaller cells later in the year giving the appearance of 'growth' rings for each year. Trees also grow taller before they grow thicker. If you drive a nail into a tree at 4 feet, when that tree reaches 100 feet tall, the nail will still be at the 4 foot mark but will likely be covered up by layers of vascular cambium and secondary growth.

Roots

In order to look at root anatomy, we need to begin at the tip. At the tip of most roots is a root cap which is dome shaped. As this root cap moves through the soil, the soil particles will damage and scrape off cells at the surface of the root cap. The damaged cells will slough off and cover the rest of the root with a protective slime that facilitates penetration through the soil. I have found nothing in my research that indicates that roots split when encountering sharp objects in the soil. This had been one of the ideas put forth in some Bonsai related publications as another reason for using sharp stone as drainage material in the soil components.
There is an apical meristem at the end of each root which replaces the damaged root cap cells. The root apical meristem is a disk of cells just above the root cap that produces new cells from it's surface. The cells from the lower surface of the disk become root cap cells while the cells on the upper surface elongate which pushes the root tip downward through the soil. The elongation of the cells on the upper surface comes from the zone of elongation which is one of the 3 general regions of a root. The cell division layer or root cap is followed by the zone of elongation. A zone of maturation behind the elongation area allows cells to develop into their specialized roles as dermal, ground or vascular tissue.
If we look at a cross section of a root in the maturation area, we will see a single cell thick layer of protective epidermis (dermal or skin) which absorbs water and minerals. Tiny root hairs extend this capacity for absorption. These tiny hairs are extensions of the epidermal cells and they exist by the thousands on each healthy root. A single ryegrass plant can have ~14 billion (that's BILLION) root hairs which combine to form an absorptive area the size of a tennis court. Let me know when you're surprised.
Of course, root hairs do break off and they are short lived anyhow. Koreshoff states that it's only the roots that are less than 2 weeks old that actually do the feeding. The other roots basically act as conduits for nutrients and water. The good thing is that root hairs are continually produced in the zone of maturation to keep the process going. If the bonsai soil dries completely out, the process of growing root hairs has to begin all over again. This is also the reason why fertilizer balls are applied to the edges of pots where the feeder roots are usually located.
Secondary growth in roots (thickening) occurs from lateral meristems just as in other plant tissues. Tips grow downward from the apical meristems and outward resulting in the root system becoming woody, stronger and capable of anchoring large trees. The vascular and cork cambiums of roots expand to produce new layers of phloem (on the outside) and xylem (on the inside). Xylem (non-living, large cells) create what we recognize as wood. Trees in nature continue growing until the epidermis and cortex layers rupture, generating new waterproof layers of cork to protect the root. The cylinders of vascular and cork cambium extend into the stem and generate wood, bark and more secondary growth. And so it goes.
Next time we will explore growth of the above ground shoots, leaves and leaf tissues. Have fun.