Intricacy is that which is given from the beginning, the birthright, and in intricacy is the hardiness of complexity that ensures against the failure of all life. This is our heritage, the piebald landscape of time. We walk around, we see a shred of the infinite possible combinations of an infinite variety of forms. --Annie Dillard, Pilgrim at Tinker Creek
1. Introduction. We will now focus on structure and physiology of the flowering plants. In this lecture I will introduce certain aspects of the various plant tissues, and will begin my coverage of plant organs with the stem. I should mention two things at this point: first, I will be going over a lot of new terms for you - you should make every effort to learn this terminology. Having a command of it will be important to you in several ways, especially when you are thinking about the plant system as a whole. Over the quarter, I will be trying very hard to step back from the details from time to time and comment on the evolutionary and ecological roles and functions of certain anatomical features of the various groups we looked at. Learning this information will help you in your ability to see these patterns yourself.
2. General structure
Plants are either herbaceous (17167) or woody (17239). Herbaceous plants do not develop persistent woody parts above ground, whereas woody plants (trees, vines, shrubs) do. Annuals are herbaceous plants that grow, reproduce, and die in one year. Other herbaceous plants are biennials, completing their life cycles in two years (growing and storing reserves in year 1, flowering and seeding in year 2). Perennials are woody or herbaceous plants that live for more than two years. In temperate climates, the aerial stems of herbaceous perennials die back each winter. The underground part (roots, tubers, stems, corms, bulbs, etc) send out new aboveground shoots each spring. In the tropics, some do this in the dry season. Woody perennials may become dormant above ground in winter (in the temperate zone). Most are deciduous and shed their leaves. In tropics, woody things can be drought deciduous.
Roots - belowground. Shoots - aboveground. Dynamic equilibrium between the two.
3. The Three Tissue Systems. Most of the plant body is composed of the following parts: the ground tissue system, which has three types of tissues and serves as the site of photosynthesis, storage and support, the vascular tissue system which functions in conduction and support, and the dermal tissue system, which provides a protective covering. Roots, stems and leaves are referred to as organs, each of which is composed of several different tissues. The tissue systems generally form an interconnected network throughout the plant. Simple tissues are tissues composed of a single cell type (parenchyma cells, collenchyma cells, and sclerenchyma cells), whereas complex tissues are composed of two or more types of cell (xylem, phloem, epidermis, and periderm).
4. The ground tissue system is composed of three simple tissues (parenchyma tissue, collenchyma tissue, and sclerenchyma tissue). Recall that a plant cell is surrounded by a cell wall which provides structural support. A growing cell secretes a primary cell wall, which stretches and expands as the cell increases in size. Once it stops growing, certain cells then secrete a thick, strong secondary cell-wall inside the primary cell wall, but outside the plasma membrane. The types of cell walls present are important in classifying the various cell types, and thus, in the case of simple tissues, the tissue types as well.
Parenchyma tissue (simple) is composed of parenchyma cells. Found throughout the plant body it is the most common type of cell and tissue. The soft parts of a plant generally consist of parenchyma tissue. Parenchyma cells also occur in vascular and dermal tissues. In parenchyma tissue, the cells have generally thin primary cell walls. These cells serve as sites of photosynthesis, storage, and secretion. Those that perform photosynthesis have chloroplasts, while other parenchyma cells may lack chloroplasts. Materials stored in parenchyma cells include starch grains, fat droplets, water, salts and crystals. Parenchyma cells are generally living 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 tissue (simple) is composed of collenchyma cells. It is a very flexible support tissue with unevenly thickened primary cell walls only (no secondary; no lignin). 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 tissue is composed of sclerenchyma cells, which have both primary and secondary cell walls. Sclerenchyma cells are not generally living at maturity. There are two type of sclerenchyma cells: conducting and support. Sub-types of support sclerenchyma cells: sclereids are short, cuboidal cells found in seeds, nut shells and the pits of stone fruits. 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).
5. The vascular tissue system consists of two complex tissues: xylem and phloem. Of necessity, vascular tissues are continuous throughout the plant body. Xylem is the water-conducting tissue, and phloem conducts carbohydrates formed in photosynthesis throughout the plant, and give structural support.
Xylem tissue (CS, Ranunculus 20192; LS, Curcurbita, 20655) is composed of tracheids (Gymnosperms have only tracheids, 20165; 20166) and vessel members (which comprise the tracheary or conducting elements), along with some parenchyma cells, fibers and sclereids. Tracheids and vessel members are elongate cells which have secondary cell walls, and lack protoplasts (e.g., are dead) at maturity. They may be covered with pits, or have them localized at the end walls. Vessel members also have perforations, generally on the end walls (but sometimes other places). These range in form from simple to complex.
Phloem tissue (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) and sclereids. Carbohydrates are conducted in solution through the sieve-tube members, which are among the most specialized cells in nature. They are elongate cells living at maturity. They are unique in that they lack a nucleus and a clear boundary between the vacuole and the cytoplasm. They are also closely associated with companion cells, with which they share numerous plasmodesmata.
6. The epidermal tissue system forms the outer protective layer of cells covering the plant body, including leaves, flowers, fruits, and seeds. The epidermis (a tissue) is the initial outer layer of protective cells. Once plants undergo substantial secondary growth, the epidermis is replaced by the periderm. Ranunculus stem (20192); Onion (20169); Zebrina (20675).
The epidermis consists of mostly parenchyma cells, with scattered guard cells, and specialized surface cells called trichomes (which may look like hair, hooks, or other interesting shapes). In roots, the trichomes are the root hairs, which function in adsorption of water and nutrients. The outer surface of epidermal cells is often coated with a waxy cuticle, which prevents water loss, and adds protection. Guard cells generally contain chloroplasts (20171, 20228, 20229), which most epidermal tissue is transparent.
Periderm consists largely of cork or phellem, originating as cork cambium (or phellogen) cells. It is non-living at maturity, and has suberized walls that are water-proof. The cork cambium is the layer of cells which produces cork tissue on its outer side, and phelloderm (a form of living parenchyma cells) on its inner side. The periderm (e.g., the bark) is the tissue which replaces the epidermis in woody plants and some herbaceous dicots (figure 5-8). (20665, 20696, 20720) many layers of dead cork cells make bark it sloughs off from the outside.
7. Meristems. Growth by cell division in the plant body is mainly carried out by meristems. There are two main types of meristem: apical (at the apex), and lateral. The apical meristems are responsible for extension of the plant body (Root apex 20679; shoot apex 20683). Apical meristems are found primarily at the tips of roots and shoots. This type of growth is called primary growth. The lateral meristems are comprised of the vascular cambium and the cork cambium. (Spruce 20163, Tilia 20656) The vascular cambium produces secondary vascular tissue: the xylem and phloem. Cork cambium produces cork cells, non-living cells which make up the majority of the bark, and parenchyma cells. This type of growth is called secondary growth.
By convention, the outer meristematic cells are called initials; the daughter cells are called derivatives. The derivatives are still considered as meristematic tissue until they begin to differentiate. They may divide several more times before beginning to differentiate. I did not mean to mislead you: cell division is not entirely limited to apical and lateral meristems. The protoderm, the procambium and the ground meristem are partly differentiated, and they are still considered primary meristem. Growth of the plant body results from cell enlargement as well as cell division. As the cells age, enlargement becomes relatively more important in growth.
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. Basic structure. The basic structure of the shoot is shown in Figure 7-2 (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. Origin and Growth of Primary tissues of the stem. 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. Organization of vascular tissues. 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.