PLANT TISSUESPRIVATE
The tissues of higher plants can be divided into three types, plus the type responsible for the production of the other three. Meristematic tissue is responsible for the production of new cells and the growth of the plant. The new cells formed from the meristems may differentiate (specialize in structure and function) into any one of the other three types of plant tissue. The dermal (epidermis and cork) comprise the outermost layers of the plant. The ground are most variable in function and type. Some of them provide support (sclerenchyma and collenchyma), others are concerned with photosynthesis (chlorenchyma), most are simple, thinwalled cells and carry out basic metabolic functions in the plant (parenchyma). There are two kinds of vascular tissue, the xylem which transports water and dissolved minerals upward from the roots and phloem which transports food materials from the leaves to all parts of the plant. Phloem cells usually do not have as thick walls as xylem and can be distinguished from them on this basis.
The structure of a plant is largely to support the process of photosynthesis. Given all we have done in lecture about the process of photosynthesis create a small table with two columns. At the head of the first column put “inputs” (to photosynthesis) and the second “outputs/wastes” (of photosynthesis). As you work through each of the organs of a plant (how many are there?) try to connect the role that organ plays in the photosynthesis process with the management of inputs and outputs and how that function is reflected in the organs structure.
Stem anatomy. Begin by looking at a slide of "Typical monocot and dicot stems" (the dicot is the section toward the top of the slide). What is a eudicot? Differentiate it taxonomically and structurally from a monocot.
Use the eudicot stem section to identify the tissue types initially. (Use the appropriate photos from your Rust lab guide and figs. 35.9 & 35.16 from your text). Begin at the outer edge of the section. The outermost, single layer of sometimes slightly flattened cells is the epidermis; the epidermis is covered by non-cellular waxy cuticle. Collenchyma tissue occurs immediately below the epidermis for four to six cell layers; the cell walls of the collenchyma are thickened only at the corners. Two to four layers of parenchyma cells (with uniformly thin cell walls) are below the collenchyma. Sclerenchyma cells occur as bundles of fibers associated with each vascular bundle; the walls of the sclerenchyma are markedly thickened on all sides. The layers of cells - collenchyma, parenchyma and sclerenchyma - between the epidermis and vascular bundles collectively make up the region called the cortex.
Immediately below the sclerenchyma cells in each vascular bundle is the phloem. The phloem is made up of two types of cells, large diameter sieve-tube elements and small diameter companion cells. The xylem is toward the center of the stem (below the phloem) in each vascular bundle, with the thick-walled vessel elements (called tracheids in some plants) as the most conspicuous cell type; xylem also contains parenchyma cells. Between the xylem and phloem within each vascular bundle is a thin layer of vascular cambium (meristem cells responsible for making more vascular tissue- xylem towards the inside and phloem towards the outside). The vascular cambium has no distinct characteristics in these slides, although it sometimes occurs as a band of slightly flattened cells, two to four cells thick, between the xylem and phloem and may continue from one bundle to another. The tissue in the center of the stem, surrounded by the vascular bundles, is the region called the pith; it is exclusively parenchyma cells.
The monocot stem differs significantly in overall aspect from the eudicot stem, in that the vascular bundles are scattered throughout the stem instead of occurring in a ring. Examine several of the vascular bundles. The vascular bundles all display a similar pattern. Using the tissues of the eudicot stem as models, identify the tissues in the monocot stem.
Next study the cross sections of the 2 to 4-year-old woody stem of the tulip tree or tulip poplar (Liriodendron). (Pay particular attention to fig. 35.18 (pg. 726).) Here the vascular cambium has produced new (secondary) xylem and phloem. The earlier (primary) xylem has been left behind as orderly rows of cells (wood), but the primary phloem has been crushed to a thin layer which lies just beneath the epidermis. The xylem cells that form in the spring of the year are bigger than those that form in the summer and fall. The latter also have thicker walls. These differences account for the annual growth rings visible in a tree trunk.
Try hard to visualize the process taking a young eudicot to a woody stem as summarized in text fig. 35.18 (pg. 726) and visualized here.
There will also be pieces of various woods available, cut in different planes. Examine these, correlating their grains with the microscopic section.
Leaf anatomy. Examine the cross section of a leaf of lilac (Syringa) or Privet. Note the following layers:
1. Cuticle and epidermis of the upper surface of the leaf.
2. Mesophyll (literally “middle” “leaf”). This is made up of two layers, the palisade parenchyma and the spongy parenchyma. (Of what advantage are the intercellular spaces in the spongy parenchyma?). The veins (vascular bundles) are distributed through the mesophyll. The xylem here again has thicker walls than the phloem and is stained pink.
3. Epidermis and cuticle or the under surface of the leaf. Note the specialized epidermal cells called guard cells. They occur in pairs with a stoma (an opening; plural: stomata) between them. Gas exchange and water loss occur through the stomata. The guard cells are capable of swelling or shrinking, through changes in water content of the leaf, depending on the light and other conditions. When they are full of water, the stomata are open; when they lack water, the stomata close. This is an important regulatory mechanism in the leaf.
Root anatomy: Study the prepared slides of median longitudinal sections of root tips of onion (Allium). (Compare what you see with the photos in the Rust lab guide and fig. 35.12 (pg. 721) of your text.) Identify the root cap, the meristematic zone, the zone of cell elongation, the zone of maturation, and the root hairs. The meristematic zone provides all the new cells for the growth of the root.
Also examine the slides of a mature root of the eudicot, the buttercup (Ranunculus) or "Typical dicot root" slide. (Fig. 35.12a (pg. 722) in your text.) Three layers should be distinguished: the outer epidermis, from which the root hairs arise; the cortex, made up primarily of cortical parenchymal cells, which may contain starch grains (stained violet); and the stele (or vascular cylinder), a cylindrical area enclosing the vascular tissue. Note that the xylem (thickened walls, stained red) in the center, is in the form of a star. The phloem cells are located between the arms of the xylem. The vascular cambium responsible for the production of xylem and phloem is located between the two.
There is a darkly-staining layer of cells surrounding the vascular cylinder called the endodermis. The cells of the endodermis have walls with a very high wax content (a hydrophobic material). This keeps all the water passing through the structure of the root, and the materials dissolved in it, from passing between the cells of the endodermis. As a result, water must pass through the cell membranes of the endodermal cells. These cells very carefully regulated what passes through them. This “gatekeeper” function results in very careful regulation of what passes into and out of the conducting vascular tissue of the stele.
There is a thin, undistinguished-looking layer of cells just inside of the endodermis. This is the pericycle and is a form of meristematic tissue- see fig. 35.14 (pg. 723) of your text to see what kind of meristem it is.
Flower and Fruit: Check out the hopefully now familiar structure of monocot and eudicot flowers. Study the following structures, progressing from the outside inward. The outer series of green leaves (sepals), not always present, and colored, modified leaves (petals). The inner sex organs are called stamens (male) and carpel (female). Each stamen consists of a slender filament at the end of which is the anther, which produces the pollen grains. Examine some pollen under the microscope. This represents the male reproductive structure and associated structures (called the male gametophyte). The carpel consists of an enlarged basal portion, the ovary, which supports a slender tube, the style. The tip of the style is slightly flattened, forming the stigma. When the flower is receptive to fertilization, the stigma becomes sticky, helping it to retain the pollen grains. Inside the ovary are several ovules, which produce the female eggs and associated structures (called the female gametophyte).
After fertilization and formation of the embryo, the wall of the ovule hardens, forming a seed coat. In some plants, the seeds are retained in the ovary, which develops into a fruit. Examine either an apple or pear, cut in cross sections. Identify as many structures as you can and correlate what you see in the mature fruit with the structures in the flower, from which they all arise.