Experiments on the secondary structure of stems
Experiments on the secondary structure of stems
Observe the secondary structure of common plant stems through a microscope, the structural characteristics of the secondary protective tissue peridermis, the structural characteristics of three sections of wood, and an understanding of the use of a slide away slicer.
Source: Botany Test Guide, Second Edition (Botany Department, School of Bioengineering, Shihezi University).
Operation method
Observation Sections
Materials and Instruments
sunflower stems linden stems acacia stems eucommia stems pine stems aloe stems Move I. Secondary structure of the stems of dicotyledonous herbs For more product details, please visit Aladdin Scientific website.
Saffron stain
Slide Microscope
1. Take a transverse preparation of a sunflower stem with thickened growth and observe the secondary structure of the stem.
(1) Epidermis: Older stems of sunflower still maintain an epidermal layer. The epidermal cells are neatly arranged on the transverse prints and are a protective tissue of plate-like rectangular cells.
(2) Cortex: Within the epidermis are cortical cells. The cortical cells near the epidermis are thick horn cells. These cells contain chloroplasts, and within the thick angular cells are imaginary layers of thin-walled cells, but not many layers.
(3) Vascular column: Outside the phloem there are heaps of thick-walled cells with distinctly lignified cell walls, which become fiber cells and are stained red by fenugreek during the preparation, these are phloem fibers.
In transverse sections of sunflower stems, it was clearly observed that the cells between the phloem and the xylem were long and flattened in shape, i.e., the cells of the formation layer, which were not a layer, but had three to five or even more layers, and could be observed to have become active. This is the formation layer in the bundle. The result of the activity of the formation layer is to divide inward to differentiate the various cells of the xylem, and to divide outward to differentiate the various cells of the phloem. From the section, it can be seen that there are more xylem cells divided inwardly than phloem cells divided outwardly.
In cross sections, relatively small vascular bundles can also be seen, which are the result of the activity of the interbundle-forming layer consisting of a portion of interbundle rays of thin-walled cells regaining their ability to divide. Pith rays are still present between the larger and smaller vascular bundles, although they are already very narrow. Within the vascular bundles there are also transverse vascular rays composed of thin-walled cells.
The sunflower has a distinct pith and pith rays present, especially the pith, which is always well developed.
2. Secondary structure of woody herbaceous stems of cotton of Dicotyledonous plants.
II. Secondary structure of woody herbaceous stems of Dicotyledons
A transverse section of a three year old linden stem was taken and its secondary structures were observed.
1. Epidermis: In the outermost layer of the stem, it consists of a layer of closely arranged epidermal cells. The cells are slightly flattened rectangular in transverse section with keratinized outer wall and cuticle. On three-year-old branches, the epidermis is no longer intact, and in some places it has been swollen and detached by lenticels.
2. Lenticels: In the cortex within the epidermal cells, many thin medullary cells, called replenishing cells, arising outward from the cork-forming layer can be observed. These cells cause the epidermis to gradually bulge and break down, which is known as lenticels on linden branches. Most lenticels are formed at the stomata of the epidermis.
3. Periderm: It is evident in the transverse sections of linden stems and consists of a combination of cork layer cells, cork-forming layer cells and inner cork layer cells.
(1) Cork-forming layer cells: These are formed after the cortical cells have regained their ability to divide. The cork-forming layer of linden stems arises from the thin-walled cells of the cortex against the outer layer. The cork forming layer cells in cross-section flat, dense cytoplasm, only a layer of cells, from which it splits outward, differentiated cork layer cells, just formed by the cork layer cells are living cells are also flattened, and cinnamon forming layer of the cells is difficult to distinguish. It looks similar to the cork forming layer.
(2) Cork layer cells: observed in cross-section, are a number of neatly arranged in the same radius line of flat cells. The wall corky thickened, is dead cells.
(3) Inner cork layer cells: the cork forming layer cells split inward, differentiated into, generally only 1 to 2 layers. Observed from the transverse section, these two layers of cells are living cells with nuclei, cytoplasm is very thick, often stained into a darker color.
4. Cortex: Outside the vascular column, it consists of only a few layers of thin-walled cells, some of which can be observed to contain clusters of crystals in the preparation.
5. phloem: outside the lamina propria, the cells are arranged in a trapezoidal shape, with their bottom edge close to the lamina propria. Fibroblasts in transverse sections are clusters of thick-walled cells stained red, and at intervals from the fibroblasts are sieve tubes, companion cells, and thin-walled cells of the phloem. The sieve tubes are generally of large caliber and the companion cells of small caliber. Interspersed with the phloem are medullary rays. These medullary ray cells grow larger the closer they are to the exterior, forming an inverted trapezoid with the cortex as the bottom edge.
6. Formation layer: The formation layer is only one layer in the fine thorax, but because the cells that split out have not yet differentiated into the various types of cells in the xylem and phloem. Therefore, it looks like this flat rectangular cell has about 4-5 layers, under high magnification, spindle primitive cells and ray primitive cells, can be observed in the transverse surface of the cell.
7. Xylem: within the formation layer, occupies the largest area in the cross section, due to the cell diameter of large and small and cell wall thickness, can be seen in the boundaries of the annual rings.
Annual rings: In a cross-section of a three-year-old linden stem, three concentric rings of annual rings can be clearly seen in the essential part. In the low magnification microscope can be observed in the fall to form a layer of cell division differentiation of the xylem various cell caliber is small, thick cell wall, especially the xylem duct molecules are more obvious, in the preparation of red staining coloring is deeper. This part of the xylem is called latewood. Spring formation of various types of xylem cells, the caliber of the conduit is large, the brain wall appears relatively thin, this part of the xylem is called earlywood. Between the latewood of the previous year and the earlywood of the next year, the boundary is often obvious, called the annual line, in the linden stems of the earlywood and latewood in cross-section, the caliber of the conduit size difference is very obvious.
8. Pith: Located in the stem and composed of thin-walled cells. It occupies a very small part of the stem cross-section. On the outside of the pith, immediately adjacent to the primary xylem, there are several layers of closely spaced thin-walled cells. Smaller volume of thin-walled cells, these cells are rich in storage material, some contain mucus, darker staining in the preparation, called ring myelin sheath. In the medulla, some cells contain crystals. Some cells are round and polygonal clusters of stone cells. The striated channels of these clusters of stone cells can be observed clearly under high magnification, and the cell lumen of the stone cells is very small.
9. Pith rays: the thin-walled cells of the pith are arranged radially outward, and when passing through the xylem, they are in two rows of cells; in the phloem, the thin-walled cells become larger and prolonged in the tangential direction, and are inverted trapezoidal.
10. vascular rays: within each vascular bundle, consisting of thin-walled cells transported laterally between the xylem and phloem, generally shorter than the medullary rays. Pith rays are present in the primary structure and they are rays between the vascular bundles. The number of vascular bundles in the stem is fixed from day to day, and therefore the number of pith rays is also fixed. Pith rays and vascular rays have different origins. Pith rays originate initially from the basic meristematic tissue; after secondary growth, they originate from the ray primordial cells. The vascular rays only come from the division and differentiation of the small ray primitive cells of the secondary meristem.
Vascular Formation Cells (Demonstration)
Take the vascular layer of the stems of acacia and eucalyptus plants. Observe the two types of cells under the microscope and identify the morphology and arrangement of the spindle-shaped primitive cells and ray primitive cells.
Fourth, the secondary structure of gymnosperm stems
Gymnosperms carry out secondary growth of stems through the production of vascular formation layer buckling periderm. The vascular formation layer constantly inward secondary xylem, outward division and differentiation of secondary phloem, thus thickening the stem year by year.
1. Observation of three sections of pine stem:
Take a section of pine stem about l0 cm in diameter three section specimen, note, cut show radial longitudinal and tangential longitudinal section of two sections, observation:
(1) Transverse section: note the thickness and color of the bark. What parts of cells does it consist of? Inside the bark is the wood, note the annual line, sapwood and heartwood.
(2) Radial section: note the direction of the annual rings, how is the pattern formed on the wood?
(3) Tangential section: note how it differs from a radial section and how it differs from a transverse section?
2. Structural characteristics of three sections of pine stem wood:
Take three sections of pine stem wood and observe: in thirty different sections, the distribution of various tissues of secondary xylem (wood) and morphological characteristics, so as to establish a three-dimensional concept of stem structure.
(1) pine wood cross-section: observe the morphological characteristics of various components; tubular cells are quadrilateral or polygonal, with ciliated pores in the radial wall of the tubular cells in profile; wood rays are arranged radially, only a column of cells wide, is a rectangular thin-walled cells; resin tracts clear amount of visible, was a cross-section.
(2) pine wood radial longitudinal section: the visible tube thorax was longitudinally arranged, the cells long shuttle, the cell wall of the ciliated pores was a frontal view; wood rays cells were longitudinally cut, transversely arranged, can be seen on the wall of a single perforation perpendicular to the axis of the stem surface, can be seen in the height of the rays; the resin tract was more longitudinally distributed.
(3) pine wood tangential section: tube cells are longitudinally arranged, the wall of the ciliated pores, a sectional view; wood rays are transverse state, the outline of the pike; can see the height and width of the wood rays.
V. Secondary structure of dicotyledonous stems (demonstration)
Take softened 2-8-year-old poplar stems, pine stems, pear stems, or peach stems, and take fresh material directly and slice it on a slide away slicer. You can also send the cut cross sections to your classmates for simple staining to observe the secondary structure of the stems.
VI. Formative Layers of Few Monocotyledon Stems Activity (Demonstration)
Monocotyledonous plants such as Aloe vera. Viburnum, Silene, Lobelia, etc., have secondary thickened growth in addition to the primary structure, thus giving rise to secondary structures. Aloe vera stem transverse section with bare hands, you can observe the epidermis, cortical thin-walled cells, between the cortical thin-walled cells and the primary vascular bundles, there are several layers of flattened cell layers, which are formed by the thin-walled fine thorax to restore the ability to divide and form the formation of layer cells. These cells mainly divide tangentially, producing a small number of thin cells outward, and the cells produced inward develop into secondary vascular bundles. The secondary vascular bundles, like the primary vascular bundles, are dispersed in the thin-walled cell tissues.
VII. Secondary protective tissues - periderm and lenticels
The epidermal tissues of the stems of most herbaceous plants play a protective role throughout the life of the plant. However, the epidermal tissue of woody plants can only be maintained for a period of time before it is shed. Therefore, a new protective tissue, called periderm, is often formed before the epidermis is shed to accommodate the thickening of the stem.
1. Take a branch of elderberry and observe the pericarp and lenticels on the branch.
2. Observe the internal structure of the pericarp and lenticels: take a transverse section of an elderberry stem through a lenticel and observe it under a low magnification lens. Firstly, a rupture with two arched sides was found, which was the lenticel. And the two sides of the lenticel are the pericarp.
Changing to high magnification, the structure of pericarp was first observed in detail, which was divided into three layers:
(1) cork layer: in the residual epidermis (dead cells), for the multi-layer according to the radius line neatly arranged cells. There is no cell space, cell wall thickening and embolization, no living material in the cell, no nucleus, is a dead cell. So the cork layer is neither water permeable nor air permeable.
(2) cork-forming layer: inside the cork layer, is a layer of flat cells, thin-walled, with a nucleus, with thick cytoplasm, can be split outward cork layer (note that it is not all of a sudden on the hydropower embolization), inward splitting embolism layer.
(3) Embolus layer: immediately after the cork layer, generally 1-2 layers of living cells, with a clear nucleus and cytoplasm, thin-walled, no embolization. According to it according to the radius line neatly arranged characteristics, can be distinguished from cortical cells.
The periderm is wrapped around the outside of the old stem and performs a protective function instead of the epidermis. Secondly, if we look at the lenticels on the pericarp, we can see that on the inner side of the rupture is a group of loosely arranged, round, thin-walled cells, called supplementary cells. The cell gap is very large, so here can be breathable. On the inner side of the supplementary cells, there is also a cork-forming layer and an inner layer of cork.
Since the cork layer does not produce a cork layer here, but produces a large number of complementary cells, it can break through the outer epidermis and the cork layer and form a fissure-like lenticels.