Plant Anatomy: A Practical Guide - Panyuta O.O. 2019

Topic 2. Tissues
Laboratory Work No. 10. Assimilation Tissues

Theoretical Background. Tissues whose primary function is Photosynthesis are known as assimilation tissues (or assimilatory tissues). They consist of parenchymal Cells with thin Cellulose walls. Due to their green coloration, they are also referred to as chlorenchyma.

Although assimilation tissue can be found in many plant Organs (with the exception of the ROOT), its primary photosynthetic function is carried out by the chlorenchyma of the green leaf. This tissue is heterogeneous and is divided into columnar (or palisade) parenchyma and spongy parenchyma. The palisade parenchyma lies directly beneath the upper epidermis of the leaf. Its cells are elongated and cylindrical, arranged so that their longitudinal axes are parallel to one another and perpendicular to the leaf surface. The Cells of the palisade parenchyma fit together loosely, with intercellular air spaces located between their elongated walls. In some plants, the palisade parenchyma may consist of only a single layer of cells, whereas in others, it comprises two or even three layers rich in METABOLISM/14.html">Chloroplasts.

Beneath the palisade parenchyma lies the spongy parenchyma, which consists of isodiametric cells that may be rounded, lobed, or stellate-lobed. This tissue is termed 'spongy' because of its loose Structure. Its intercellular spaces form a complex network that allows air to circulate freely in all directions. Chloroplasts are also present in the spongy parenchyma, though in significantly lower numbers than in the palisade layer. As a rule, the palisade parenchyma is located on the upper side of the leaf, while the spongy parenchyma is on the lower side.

In certain plants, such as cereals and conifers, the chlorenchyma has a specific structure. Its Cell walls form inward folds projecting into The Cell lumen, which significantly increases the total surface area of the walls and allows a greater number of chloroplasts to be accommodated along them.

In addition to the palisade and spongy parenchyma, the assimilation process can also take place in other chloroplast-bearing cells. For instance, the synthesis of organic substances can occur in the leaf epidermis within the guard cells of Stomata, in the primary cortex of the stem, and even in aerial roots. However, the assimilatory activity of these cells is not their primary function, and they lack the typical structure of assimilation tissues. The best object for studying The structure of assimilation tissue is the leaf of any green plant.

Objective: to examine the Structural Features of assimilation tissues.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass rods, filter paper, distilled Water, plant material.

Slide. Assimilation tissues in the leaf of wild radish (Raphanus raphanistrum L.)

Take a fresh green leaf of common radish and make a cross-section capturing both the upper and lower surfaces. Because a radish leaf is very soft and difficult to section freehand, it is helpful to use a piece of elderberry pith. Carefully make a longitudinal slit in the pith using a razor blade and insert the strip of leaf into the cleft. Hold the pith containing the leaf firmly between the fingers of your left hand, and slice off the portion protruding above The surface of the pith. Having leveled the plane so that the cut is not slanted, make several simultaneous cross-sections through both the pith and the leaf. Discard the elderberry pith, place the leaf sections in a drop of water on a Microscope slide, cover with a coverslip, and examine first under low magnification (to select the thinnest area of the section) and then under high magnification.

The upper side of the leaf is covered by a layer of epidermal cells. The lateral and inner walls of these cells are thin, while the outer wall is slightly thickened. The living contents are not visible within the epidermal cells (Fig. 56).

Class="center">

Fig. 56. Cross-section of a radish leaf:

1 - sclerenchyma, 2 - xylem vessels, 3 - stoma, 4 - cambium, 5 - fundamental parenchyma, 6 - phloem, 7 - lower epidermis, 8 - spongy parenchyma, 9 - palisade parenchyma, 10 - upper epidermis

Beneath the epidermis lie three layers of columnar, or palisade, parenchyma. The palisade cells are elongated and thin-walled. A thin layer of Cytoplasm lines The Cell wall, containing The Nucleus and chloroplasts, which are arranged in a single layer along the cell walls. Intercellular spaces extend between the rows of palisade cells across the thickness of the leaf, allowing air to circulate freely to the cells.

Underneath the palisade layer lies the spongy parenchyma, whose cells have a lobed shape. The cell walls of the spongy parenchyma are also thin and made of cellulose, with a peripheral layer of cytoplasm lining the walls and enclosing the nucleus and chloroplasts.

The spongy parenchyma is permeated by numerous intercellular spaces that lead to the substomatal air cavity located beneath the stoma on the lower side of the leaf. Consequently, air passes freely through the stomatal pore and the intercellular spaces of the spongy parenchyma to reach the palisade parenchyma, ensuring the delivery of carbon dioxide necessary for chloroplast photosynthetic activity. In the middle of the slide, a vascular bundle is visible, comprising xylem, phloem, and cambium. The bundle is surrounded by sclerenchyma cells.

Review Questions

1. WHAT IS A tissue?

2. How are plant tissues classified?

3. What is the Anatomical and physiological Classification of plant tissues based on?

4. Name the MAIN TYPES OF plant tissues.

5. What is a growth cone (apical meristem)?

6. Prove that the SHOOT apical meristem is primary in origin.

7. What are the Main Functions of apical meristem cells?

8. What is the difference between apical and intercalary Meristems?

9. Which meristems are called primary and which are secondary?

10. Name the types of lateral meristems.

11. Name the Primary and secondary meristems.

12. To which tissue group does the epidermis belong? Justify your answer.

13. What groups of cells make up the epidermis?

14. What are the functions of stomata?

15. What is The Significance of the uneven thickening of the cell wall in guard cells for stomatal function?

16. What structural Features of the epidermis provide its protective properties?

17. What are trichomes, and in which tissue are they located?

18. In what ways are stinging and glandular hairs similar and different?

19. What Properties of the secondary dermal tissue ensure its protective function?

20. Which cell layer of the stem forms the phellem and phelloderm?

21. Name the structures of the periderm through which gas exchange occurs between internal living tissues and the external environment.

22. How to distinguish between epidermal and cork cells?

23. Name the types of mechanical tissues.

24. What common feature is characteristic of mechanical tissues?

25. Describe the structural features of collenchyma cells.

26. What is The Role of collenchyma in the stem?

27. Name the types of collenchyma.

28. Describe the structural features of sclerenchyma cells.

29. What is the mutual arrangement of collenchyma and sclerenchyma in a pumpkin stem?

30. Which tissue provides the upward water flow, and which provides the downward flow in a plant?

31. What histological elements make up the xylem?

32. What histological elements make up the phloem?

33. What types of vascular bundles are distinguished based on the relative arrangement of phloem and xylem?

34. What are collateral and bicollateral vascular bundles, or how are closed and open vascular bundles defined?

35. What type of vascular bundles is present in the stem of a pumpkin?

36. What type of vascular bundles is present in the stem of corn?

37. What are laticifers, and to which type of tissues do they belong?

38. How do assimilation tissues differ from other tissue types?

39. Name the main types of assimilation tissues and state where they occur.

40. Which epidermal structures contain chloroplasts?