Lecture 36 April 28 2003 R.Jones Chapters 31 & 7

Plant Tissues, Wood, Growth Rings, Bark. Begin Photosynthesis

1. Enclosed within the eipdermis is the ground tissue in which the vascular tissue is embedded. The ground tissue of the shoot is divided into cortex and pith and in most roots only a cortex is found. The ground tissue consists of three cell types. Parenchyma, involved in photosynthesis or storage, collenchyma specialized by cellulosic thickenings that give strength (found in celery petiole "strings") and sclerenchyma that are lignified cells. Sclerenchyma can be elongated and called fibers as in the fibers from flax from which linen is made or the long fibers of hemp from which rope is made. The thickenings of fibers are of lignin. Sclereids are another form of sclerenchyma and these cells are often branched and give the gritty texture to pears.

2. Embedded in the ground tissue are the vascular tissue of the plant made up of xylem and phloem. Xylem and phloem are associated together in something called a vascular bundle. The xylem has two types of cells, the tracheids and vessel members that are involved in water and mineral transport upwards from soil (roots) to the atmosphere via the shoot. These cells have cellulose and lignin in their walls and they are dead when functionally mature.

3. Phloem cells are involved in transport of organic solutes, e.g., sugars and amino acids. They consist of two specialized cell types, the sieve element or the sieve cell and the an associated companion cell. These cells are alive at functional maturity indeed they will not function when dead. They can transport materials both up and down the plant body.

4. The orgnaization of the tissues in the root, stem and leaves are illustrated in your textbook in Figures 31.6 (stem).

5. Secondary growth of the plant body allows for the plant to increase in size radially and is brought about by the LATERAL MERISTEMS of which there are two. The VASCULAR CAMBIUM gives rise to secondary phloem and secondary xylem, wheres the CORK CAMBIUM gives rise to cork. Secondary growth is found in most woody dicots and gymnosperms but is absent from many monocots except woody monocots such as magnolia. Plants such as corn have no lateral meristems and no secondary growth. Similarly, tall palms lack secondary growth and are supported by leaf bases.

6. In the shoot, the vascular cambium arises in the vascular bundle between the phloem and xylem and this meristem divides to produce secondary xylem to the inside of the shoot and secondary phloem to the outside of the shoot. The cork cambium arises in the outer layers of the cortex and gives rise to cork to the outside and layers of parenchyma-like cells to the inside called phelloderm.

7. Bark is a lay term to indicate all of the tissues outside the vascular cambium in a woody stem or root. The vascular cambium represents a weak spot of small dividing cells that can be easily broken. When bark is removed so is the phloem which lies to the outside of the vascular cambium. Because phloem transport sugars and other products of photosynthesis from shoots to roots, removal of bark from a complete circle around the stem with eventually result in death of the plant roots and the plant.

8. The vascular cambium and cork cambium are collectively responsible for Secondary Growth. Secondary growth allows for the build up of wood. Wood is seccondary xylem that accumulates because the tracheids and vessels are highly lignified and are not crushed as the stem or root grows. The scondary phloem cells are alive and must reamin alive to function. Because they do not have rigid thick walls they do not contribute to wood formation.

9. The vascular cambium in temperate plants undergoes seasonal cycles of cell division. Cell division ceases in the fall and resumes in the spring. When growth resumes cells tend to be large and when growth stops in the fall cells tend to be small. These differences in cell size cause the growth ring to be demarcated. Some woods also make vessels in the spring causing the wood to become even more characteristically different.

10. Photosynthesis: The process whereby solar energy is converted to chemical energy. Broken down into two distinct sets of reactions-light reactions that result in the formation of ATP and NADPH and dark reactions where ATP and NADPH function in endergonic reactions to synthesize sugars.

11. History of photosynthesis is fascinating since it tells much about scientific thinking in the 17th and 18th centuries. The Belgian priest van Helmont did an experiment where he grew a 5 lb sapling in 200 lb soil and watered it daily. After 5 years growth he weighed the soil (199 lb 14 oz) and the sapling(169 lb) and concluded that plants grew from water because the plant added 164 lb in weight and the soil lost 2 oz, a perfectly reasonable conclusion at the time.

12. Stephen Hales (1677-1761) and Joseph Priestley (1733-1804) were two 18th century scientists, both clergymen, who made important early discoveries about photosynthesis. Hales showed that plants changed the composition and volume of air in which they grew and Priestley discovered that plants could renew the atmosphere in which a candle was burned, allowing animals to survive in the air that was purified by plants.

13. The Dutch physician Jan Ingenhousz (1730-1809) was the first to show that light was involved in this process of air purification when her reported that light, and bright light-bright sunlight after dawn and before dusk and not in shade, was needed for plants to purify air and he further showed that leaves and green stems but not other parts of the plant were involved in air purification. CO₂ and O₂ were still not identified as being involved although CO₂ was discovered by the English chemist Joseph Black in 1755.