The Plant Body

1. The Plant Body

3. Fun plant facts • There are over 320,000 species of plant - they differ from each other in light gathering shoots and water/nutrient gathering roots, but they also differ from each other in terms of biochemistry • Because plants can’t move they have difficulty reproducing and evading predators • Plants produce many different chemicals to prevent or at least minimize animal predators - many of these defensive chemicals are used by us for medicines – 80% of all people get their medicines directly from plants - 25% of our prescription medicines come from plant chemicals

4. Commonly used and abused chemicals substances from plants • caffeine in coffee • theophylline in tea • theobromine in chocolate • nicotine in tobacco • cocaine from coca plant leaves • THC in Cannabis • and many other narcotic and psychoactive substances

6. Cooksonia – 408 MYA

7. Basic plant structures

9. Plant Tissue Types • Meristematic tissue - site of growth in plant; origin of the other tissue types • apical meristems - site of primary growth; lateral meristems - site of secondary growth • Dermal tissue system - the outer protective covering of the plant • Vascular tissue system - comprises the xylem and phloem - it is embedded within the ground tissue system – xylem forms wood in woody plants • Ground tissue system - the inner supportive tissues of the plant – pith in herbaceous plants

10. Primary Plant Growth • Primary growth originates in the apical meristems and results in increases in length - tissues originating from primary growth make up the primary plant body (primary xylem, primary phloem, etc) - many vascular plants consist entirely of primary tissues

12. Secondary Plant Growth • Secondary growth originates in the lateral meristems and results in increases in width - there are two lateral meristems • Vascular cambium which produces secondary vascular tissues - secondary xylem to the inside and secondary phloem to the outside • Cork cambium is outside the vascular cambium - it forms periderm, which is made of cork tissue - the periderm replaces the epidermis as the dermal tissue system of the plant

14. Vascular Tissues • Secondary xylem is dead at maturity and transports water essentially through a hollow tube - angiosperms have tracheary cells are called vessels and tend to have flattened ends, as well as tracheids • in gymnosperms the tracheary cells are called tracheids and usually sharply tapered • eventually the xylem becomes full of sap and is no longer used for water transport, then it functions in support and forms wood • Phloem cells are called sieve tube elements because of the sieve like plates at the end of the cells - they are alive at maturity but are crushed as the plant grows in diameter and must be continually replaced • Some sieve cells have companion cells which govern transport of material through the sieve

15. Xylem – Vessels and Tracheids

16. Phloem – sieve elements

18. Plant growth • Plant material originating from secondary growth forms the secondary plant body • Secondary growth appeared in the Devonian about 380 MYA • Many plants grow continuously throughout their life and have no final particular size or shape - this is called indeterminate growth • Many plant parts such as leaves, flowers and fruit do have a characteristic final shape and size and stop growing at that size - that is called determinate growth

19. Garlic Roots

21. Root vegetables

22. Fibrous Tap

23. Varieties of Root Systems

24. Soil root traces

27. Lateral growth of a willow root

28. Back to the shoot system

29. Plant Embryo

30. Apical Dominance • Usually the growing terminal bud inhibits the development of the lateral buds, a phenomenon known as apical dominance - as the distance between the shoot tip and lateral buds, the influence of the apical meristem lessens and the lateral buds proceed with their development

31. Apical Dominance

32. Bud Pinching

33. Stem Function The two main functions of the stems are conduction and support • Conduction involves moving substances manufactured in the leaves through the phloem to other parts of the plant including developing leaves, stems, roots, developing flowers, seeds and fruits and the xylem carries water from the roots to the leaves, where water is transpired • Support involves holding the plant off the ground - supporting the principal photosynthetic organs of the plant (the leaves) as well as flowers, seeds and fruits

35. Stem Growth • The organization of the apical meristem of the stem is more complex than is the organization of the apical meristem of the root • The apical meristem adds cells to the plant body and forms leaf primordia and bud primordia that develop into lateral branches • The apical meristem of a stem lacks a protective cover like the root cap of roots

36. Stem Growth cont’d • Protoderm always originates from the outermost meristem cell layer • Procambium and part of the ground meristem (which will form the cortex and sometimes part of the pith) form from the peripheral meristem • The rest of the ground meristem (which forms some or all of the pith) is formed by the pith meristem

38. More Stem Growth • Usually the meristematic activity causing the elongation of the internodes is most intense at the base of the developing internodes - if elongation of the internodes occurs over a long period, the meristematic base of the internode may be called an intercalary meristem (a meristematic region between two highly differentiated regions) – intercalary meristems are very important in the growth of grasses and grass-like plants

42. Three types of primary structure 1. In some conifers and dicots, the narrow, elongated procambial cells (and consequently the primary vascular tissues that develop from them) appear as a more or less continuous hollow cylinder within the ground tissue - the outer region of the ground tissue is called the cortex and the inner region is the pith

43. Basswood stem cross-section

44. Basswood – 2 year old stem

45. Three types of primary structure 2. In other conifers and dicots, the primary vascular tissues develop as a cylinder of discrete strands separated from one another by ground tissue • The ground tissue separating the procambial strands (and later mature vascular bundles) is continuous with cortex and pith and is called the interfascicular parenchyma (between the bundles) • The interfascicular regions are often called pith rays

46. Sunflower stem cross-section

47. Comparison of Dicot Stems

48. Three types of primary structure 3. In most monocots and some herbaceous dicots, the arrangement of procambial strands and vascular bundles is more complex - vascular tissues do not appear as a single ring, but instead develop as more than one ring or are scattered throughout the ground tissue - here ground tissue cannot be distinguished as pith and cortex - often called pith

49. Corn stem cross-section

50. Monocot stem with labels