Entry Three: Plant Growth

Growth occurs throughout a plant’s life, a process known as indeterminate growth. Most plants grow continuously, however, some plants organs, such as leaves, thorns, and flowers, undergo determinate growth – that is, they stop growing after reaching a certain size. Plants are capable of indeterminate growth because they have perpetually undifferentiated tissues called meristems that divide when conditions permit, leading to elongation. There are two types of meristems: apical meristems and lateral meristems. Apical meristems, located at the tips of roots and shoots and in axillary buds of shoots, provide additional cells that enable growth in length, a process known as primary growth. Growth in thickness, however, is known as secondary growth and is caused by lateral meristems called the vascular cambium and cork cambium. These cylinders of dividing cells extend along the lengths of roots and stems. The vascular cambium adds layers of vascular tissue called secondary xylem, or wood, and secondary phloem. The cork cambium adds secondary dermal tissue, replacing the epidermis with the thicker, tougher periderm. Although plants grow throughout their lives, they die like any other living things. Based on the length of their life cycle, flowering plants can be categorized as annuals, biennials, or perennials. Annuals complete their life cycle in a single year or less. Biennials require two growing seasons to complete their life cycle. Perennials live for many years.

Primary growth lengthens roots and shoots. Although the elongation of both roots and shoots arises from cells derived from apical meristems, the primary growth of roots and primary growth of shoots differ in many ways.

Let’s talk about the primary growth in roots first. The tip of a root is covered by a thimble-like root cap, which protects the delicate apical meristems as the root pushes through the soil during primary growth. Growth occurs behind the cap in three overlapping zones: the zones of cell division, elongation, and differentiation. The zone of cell division includes the root apical meristem and its derivatives. New root cells are produced in this zone, including cells of the root cap. The zone of elongation lies a few millimeter behind the tip of the root. In this zone, root cells elongate – sometimes to more than times their original length. As it elongates, the root apical meristem keeps adding cells to the younger end of the zone of elongation. In the zone of differentiation, cells complete their differentiation and become distinct cell types. The primary growth of a root produces its epidermis, ground tissue, and vascular tissue. Lateral roots arise from the pericycle, the outermost cell layer in the vascular tissue.

Now let’s talk about the primary growth in shoots. The shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip. Shoot elongation is due to the lengthening of internode cells below the shoot tip. Branching, another part of primary growth, arises from the activation of axillary buds, which also contains a shoot apical meristem. Leaves develop from leaf primordia, finger-like projections along the sides of the apical meristem.

Secondary growth increases the diameter of stems and roots in woody plants. As mentioned before, secondary growth consists of the tissues produced by the vascular cambium and cork cambium. The vascular cambium adds secondary xylem (wood) and secondary phloem, thereby increasing vascular flow and support for the shoots. The cork cambium produces a tough, thick covering consisting mainly of wax-impregnated cells that protect the stem from water loss and from invasion by insects, bacteria, and fungi.

Let’s look at the vascular cambium and secondary vascular tissue in details. Specifically, the vascular cambium is a cylinder of meristematic cells, often only one cell thick. It increases in circumference and also adds layers of secondary xylem to its interior and secondary phloem to its exterior. In this way, the vascular cambium thickens roots and stems. Viewed in cross section, the vascular cambium appears as a ring of stem cells, or initials, and marks the annual growth of woody plants. Some initials are elongated and are orientated with their long axis parallel to the axis of the stem or root. They produce the water-conducting cells in xylem and the sugar-conducting cells in the phloem. Other initials are shorter and are oriented perpendicular to the axis of the stem or root. They produce vascular rays, radial files of mostly parenchyma cells that connect the secondary xylem and phloem. The cells of a vascular ray move water and nutrients between the secondary xylem and phloem, store carbohydrates, and aid in wound repair.

Now let’s look at the cork cambium and the production of periderm in details. During the early stages of secondary growth, the epidermis is pushed outward, causing it to split, dry, and fall off the stem or root. It is replaced by two tissues produced by the first cork cambium, a cylinder of dividing cells that arises in the outer cortex of stems and in the outer layer of the pericycle in roots. One tissue, called phelloderm, is a thin layer of parenchyma cells that forms to the interior of the cork cambium. The other tissue consists of cork cells that accumulate to the exterior of the cork cambium. Each cork cambium and the tissues it produces comprise a layer of periderm. The thickening of a stem or root often splits the first cork cambium, which loses its meristemic activity and differentiates into cork cells. A new cork cambium forms to the inside, resulting in another layer of periderm.

Below is a diagram of a leaf structure. The epidermis is interrupted by pores called stomata, which allow the release of evaporative water and the exchange of carbon dioxide and oxygen between the surrounding air and the photosynthetic cells inside the leaf. Two guard cells regulate the opening and closing of the pores. The ground tissue of a leaf, a region called the mesophyll, is sandwiched between the upper and lower epidermal layers. Mesophyll consists mainly of parenchyma cells specialized for photosynthesis. They have two distinct layers: palisade mesophyll and spongy mesophyll. Palisade mesophyll consists of one or more layers on the upper part of the leaf. The spongy mesophyll rests below the palisade mesophyll. The vascular tissue of each leaf is continuous with the vascular tissue of the stem. Veins subdivide repeatedly and branch throughout the mesophyll. This network brings xylem and phloem into close contact with the photosynthetic tissue.