Lecture 17: Plant Morphology and Intro to Stems

1. Plants exhibit diversity in structure and life span, but there are some basic commonalities. Roots, stems, leaves, flowers, and fruits make up the plant body; these are the funtional organs. Plants can be either woody (17239) or herbaceous (17167). The root system is typically below-ground, and the shoot system is the above-ground tissue, consisting of the stem, leaves, flowers and fruits. The root and shoot system are exposed to very different environmental conditions, but exist because plants require materials from both environments.

2. The plant body is composed of cells and tissues. Tissues of plants, like animal tissues, are composed of cells which form structural unit. Some tissues are simple (composed of only one type of cell), others are complex (composed of more than one cell type). The three tissue systems of plants are the ground tissue system, the vascular tissue system (functions in conduction), and the dermal tissue system (functions in covering the body). Roots, stems, leaves, floral parts, and fruits are called organs as each is composed of all of the different tissue types. The tissue systems of the root and shoot systems are connected. This is fundamentally different from the organization of animals, wherein some organs have specialized tissues found only in them.

3. The ground tissue system is composed of three simple tissues. The ground tissue system makes up the bulk of herbaceous plants. Parenchyma, collenchyma, and sclerenchyma are differentiated based on their cell wall structure. The primary cell is laid down first and it grows with the cell. A secondary cell wall may be laid down internal to the primary cell wall. Parenchyma cells have thin primary walls. Parenchyma is a simple tissue composed of parenchyma cells. The functions of parenchyma include storage, photosynthesis, and secretion. Storage of starch, oil, water, and salts occurs in parenchyma. Parenchyma cells are alive at maturity. Examples: Ranunculus (buttercup) root (21096) with lots of parenchyma. Close-up (20249), showing parenchyma cells of root cortex with lots of starch. For pure storage, check out the starch-filled parenchyma cells of the potato (20216). Sunflower stem (20652) shows that parenchyma cells are the space-filling cells between the epidermis and the vascular tissue. Privet leaf (20208), showing palisade and spongy parenchyma (also called mesophyll).

Collenchyma cells have unevenly thickened primary walls. Collenchyma is also a simple tissue made of collenchyma cells. Collenchyma provides support in soft plant organs. Collenchyma cells are alive at maturity, and have uneven thickenings of the primary cell walls, particularly in the corners. Collenchyma tissues are the major sources of support for non-woody cells and plant parts. It is not uniformly distributed throughout the plant, but is concentrated in long strands near stem surfaces and long leaf veins. Celery strands. Collenchyma cells are generally living at maturity. Examples: Celery collenchyma SEM (20213). Collenchyma in stem of Ranunculus (20192 green).

Sclerenchyma cells have both primary walls and thick secondary walls. Sclerenchyma is a simple plant tissue composed of sclerenchyma cells specialized for support. Sclerenchyma cells have secondary cell walls rich in lignin. At maturity, sclerenchyma cells are often dead. Sclerids are short, cubical cells found in nut shells and the pits of stone fruits. Fibers are long cells found in clumps, common in the wood and bark of many angiosperms. Fibers are long tapered cells found in wood and bark. Examples: Ranunculus (20192 red), Corn (20188); Water plant (20660); Sunflower phloem fibers (20680), Pear fruit (gritty cells)(20212).

4. The vascular tissue system consists of two complex tissues: xylem and phloem which are continuous in the root and shoot systems. Xylem conducts water and dissolved materials from the root to the shoot system and provides structural support. The conducting cells in xylem are tracheids and vessel elements. In flowering plants, four types of cells make up the xylem. Tracheids and vessel elements conduct fluid. These cells are dead at maturity, and are hollow tubes. Tracheids are the major components of xylem in gymnosperms. Water passes from one tracheid to the next through pits in the cell walls. Angiosperms have tracheids and vessel elements. Vessel elements have a larger diameter and may be connected directly, end to end. A stack of vessel elements is called a vessel. Water may also pass laterally between vessel elements via pits. Xylem also has fibers and parenchyma cells which are not conducting cells

Xylem tissue (CS, Ranunculus 20192; LS, Curcurbita, 20655) is composed of tracheids (Gymnosperms have only tracheids, 20165; 20166)

Phloem conducts dissolved sugars throughout the plant, and provides support. Phloem is a complex tissue composed of two types of conducting cells, and fibers and parenchyma. Dissolved sugars are conducted in sieve tube members. Sieve tube members are stacked like vessel elements in tubes called sieve elements. Sieve tube members have sieve plates separating them. Sieve tube members are alive at maturity, but most organelles have degenerated, including the nucleus. Companion cells are adjacent to sieve tube members and direct the activities of both cells via cytoplasmic connections through plasmodesmata. (Ranunculus 20192; Curcurbita sieve tube member 20157) is composed of sieve cells and sieve-tube members (which comprise the sieve elements), along with specialized parenchyma cells called companion cells(Curcurbita 20713) (albuminous cells in gymnosperms), and other storage-related parenchyma cells, fibers (Helianthus, 20680)

5. The dermal tissue system covers and protects the plant. The dermal tissue system is a single layer in herbaceous plants, and can be very thick in woody plants. Epidermis is the outermost layer of cells of a herbaceous plant. The epidermis is a complex tissue made of parenchyma cells with a small number of guard cells and may contain outgrowths called trichomes. The epidermis is typically one cell thick; the epidermal cells have thickened cell walls on the outer margin. Epidermal cells are typically nonphotosynthetic, and are relatively transparent, allowing light to penetrate deeper tissues. Epidermal tissue retards water loss with a waxy cuticle. Stomata, surrounded by guard cells, allow diffusion of gases into and out of the leaf. Trichomes are hairlike projections which function in increasing the effective surface area of the root (root hairs), salt excretion in halophytes, and may be protective in plants like nettles. Ranunculus stem (20192); Onion (20169); Zebrina (20675).

Epidermis is replaced by periderm in woody plants. As a woody plant grows, the epidermis is lost and replaced by periderm. Periderm is a complex tissue made of cork cells and cork parenchyma cells. Cork cells are dead at maturity and function in waterproofing. Cork parenchyma cells function in storage. The periderm (e.g., the bark) is the tissue which replaces the epidermis in woody plants and some herbaceous dicots (20665, 20696, 20720) many layers of dead cork cells make bark it sloughs off from the outside.

6. Plants exhibit localized growth at meristems. Growth includes mitosis, cell elongation, and cell differentiation. Cell elongation occurs as the central vacuole fills with water. Cell differentiation involves specialization into the different cell types. Unlike animals, plants grow primarily in specialized areas known as meristems. Cells in the meristems do not differentiate, and they retain the ability to divide. Primary growth involves an increase in the length of a plant, and all plants grow this way. Secondary growth involves in the increase in the diameter of a plant, and typically only gymnosperms and woody plants exhibit secondary growth. Monocots lack true secondary growth. Some herbaceous dicots exhibit secondary growth (such as the geranium). Primary growth takes place at apical meristems. In the root of a plant, an apical meristem occurs just behind the root cap. Behind the meristematic area, the area of cell elongation is marked by cells which are growing in size and begin differentiation. In the buds on the shoot system, a dome of meristematic cells forms an apical meristem. Leaf primordia and bud primordia arise from this meristem. (Root apex 20679; shoot apex 20683)

7. Secondary growth takes place at lateral meristems (Spruce 20163, Tilia 20656) .Lateral meristems extend the length of stems and roots and have two meristematic areas. The vascular cambium is a cylinder of meristematic cells between the wood and bark of a woody plant. Cells of this cambium add to the wood (secondary xylem) and inner bark (secondary phloem). The cork cambium is a meristematic cylinder in the outer bark and cells produced here form the cork cells and cork parenchyma cells of the periderm. Bark, therefore, can be defined as a living inner bark made of secondary phloem and a dead outer bark made of periderm.

8. Introduction to stems. In general, shoots are more complex than roots in external structure so it also follows that their internal anatomy is more complex. Normally, roots only branch to produce more roots, whereas shoots must branch and differentiate to produce several different structures including leaves, flowers, shoots. The apical meristem of the shoot thus gives rise to various kinds of primordia (meristematic cells which form twigs, leaves, and flowers).

The primary functions of the stem are conduction, support, and the growth and formation of new aboveground tissue. The stem also functions as a site for storage of carbohydrates, and in leaf display.

9. The basic structure of the shoot is shown in Figure 31-3 (transparency). Leaves arise at nodes, and the longitudinal space between successive nodes is termed the internode. The shoot tip is the growing tip, and is generally subtended by young leaves or buds. Lateral buds arise in the axils of the leaves. Commonly, the growing terminal bud inhibits the development of lateral buds below it via hormonal control. This phenomenon is called apical dominance.

10. Here (20658), we see an image of a longitudinal cross section of a Coleus shoot tip. Leaf primordia (20221) arise from the apical meristem in a pattern and sequence that determines the arrangement of leaves (singly or in pairs and in one or more planes). In the axil of each leaf primordium is another meristematic area, the bud primordium. The shoot apex exhibits a tunica-corpus organization. The tunica is the layers of cells parallel to the surface; the corpus is the layer below, with cells organized more in a mass and not all dividing in the same plane. The bulk of the corpus may divide slowly, and is often called the region of central mother cells. This region divides relatively slowly, and corresponds to a similar quiescent zone in the root.

The node remains a site of meristematic potential for a long time (measured in years), and not just for shoots; roots are also more readily induced to form at nodes than elsewhere. Most stem elongation is through internode extension. Continued extension growth arises from a region near the base of the internode called intercalary meristems. These regions of the internode can continue stem elongation after meristems above them have completely matured.

Primary growth should be differentiated in your minds from secondary growth. During primary development, stems retain their epidermis and usually appear green. The stem epidermis contains stomata and may also possess hairs or trichomes. If a plant undergoes secondary growth, the epidermis is shed, to be replaced by the periderm.

11. We currently recognize three main types of organization of the vascular tissues in stems: 1) an arrangement characterized by a vascular cylinder, a continuous ring of vascular tissue (20685). This type of organization is found in some dicots and some gymnosperms. 2) an arrangement characterized by a ring of discrete bundles (20715,20219). This type of organization is also found in some dicots and some gymnosperms. 3) an arrangement in which the bundles are scattered through the ground tissue which cannot be identified as either cortex or pith(20187). This type of organization is found in most monocots and some herbaceous dicots.

In plants with the first arrangement, under the epidermis is the cortex of parenchyma cells and within this is found the continuous cylinder of vascular tissue. Inside the cylinder of vascular tissue is the pith, a mass of parenchyma cells.

In plants with the second arrangement, such as sunflower, each bundle consists of phloem on the outside and xylem on the inside of the stem. There are usually some fibers associated with the bundle which can be in a cap-like cluster outside the phloem or a sheath around the whole bundle (or both). Across the middle of the bundle is a band of small cells which are capable of cell division, the fascicular cambium. This cambium may extend across the pith rays between the bundles where it is called interfascicular cambium.

In plants with the third arrangement, such as corn, the bundles are scattered and contain xylem (inside) and phloem (outside). There are fibers around the outside and the large air-filled cavity in the mature bundle from destruction of the first formed xylem and phloem. There is no cambium and certainly no interfascicular cambium. Monocots have closed bundles because there is no possibility of further differentiation of xylem or phloem. In contrast the vascular bundles of dicots are called open, which means that they can still undergo secondary growth.

12. In primitive plants the vascular system is not as well developed, and a relative lack of connections between different parts of the plant imposes a limitation on the movement and translocation of water and food, which in turn, limits size and complexity. During their evolution, plants have developed improved techniques for linking their transport members, and in higher plants, this leads to some quite complicated plumbing. As stems mature, the vascular bundles maintain structural continuity between the leaf and the stem and between the stem and its branches. Above the point where a trace leaves the stem there is a gap in the vascular tissue which is closed by branches coming from other bundles lower in the stem.