Botany Lesson Fifteen Gymnosperms

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1 Botany Lesson Fifteen Gymnosperms In this unit, we will: Investigate key differences between gymnosperms and angiosperms, completing our table Learn about how gymnosperms reproduce Learn about wood Discover how to measure the approximate age of a tree You will need: Calculator and measuring tape Optional: wood samples of hardwood and softwood The ever green evergreen When we think of gymnosperms, we usually think of Christmas trees and pine cones. While pines are gymnosperms (part of the Conifers), there are actually four groups of gymnosperms, of which Conifers are the largest. The other three groups are: Gingko (only one species), Cycads (which resemble palm trees), and the Gnets (which include Welwitschia of West Africa and Ephedra, both shrubby plants). Some characteristics of Gymnosperms leaves are that they are needle or scale-like and they have a thick cuticle to prevent water loss, much like cacti, and many live in desert-like conditions (deserts aren t necessarily hot. Antarctica is actually considered a desert). While there are some species that lose their leaves in autumn all at once, such as tamarack, bald cypress, and larch, most are evergreen and replace their leaves sections at a time before they shed old leaves. Gymnosperms are all woody, being shrubs or trees, and their trunks have vascular bundles arranged much like the dicots of the flowering plants (the roots are also similar to dicots). One unique aspect of

2 them is that many of them produce true resin in their trunks and branches to repair damage. It is not soluble in water and hardens when exposed to the air. It is toxic if ingested and is powerful medicine. Resins are used by people as lacquers, solvents, and cleaners. While some flowering plants also produce resins, these are usually in combination with gums, mucilages, waxes, latex, or oils. Wood While wood comes from both deciduous and coniferous trees, it makes sense to talk about how trees grow in girth each year in this unit because all conifers are trees. I m sure that somewhere in your house you have some bookshelves or cabinets made of pine or oak. Maybe your floor is a hardwood floor made from oak. The fact that trees add a ring of growth each year (sometimes multiple rings in a single year) is what enables us to eventually cut the tree into boards for use in countless ways. Take a look at the diagram of a tree ring below. Remember mericells and how they are like human stem cells? Remember the vascular cambium from dicot stems and how on one side there was the xylem and on the other side was the phloem and the vascular bundles formed a ring in the stem? The vascular cambium is made up of meristematic cells that

3 divide to produce new xylem and phloem cells. The outer bark protects the tree and as it is stretched each year, it forms a unique pattern of ripples and ridges. Just beneath the bark is the phloem layer, often called the inner bark. It is very thin. Just beneath the phloem or inner bark is the thin vascular cambium layer. All of the new wood is just below the cambium and is the xylem layer. In the spring, the cambium produces larger xylem cells with thin cell walls, making the wood appear light. In the summer, the cells are denser and smaller, making the cells appear darker. This variegation in color produces the beautiful streaks and grain in wood planks when they are cut. The center of a tree is dead heartwood. It is previous years xylem and is no longer used for transporting nutrients from the roots. However, it does add rigidity to the tree. Never peel bark off of a tree! Because the essential circulatory system is so close to the bark, you can kill the tree by peeling the bark. Activity In this activity, we will try a method of dating trees without cutting them down. Go to this website: You will need: measuring tape and calculator. Activity In this activity, we are going to compare a hardwood and a softwood. You will need: a sample of pine and a sample of oak. Wood scraps are often available at lumberyards and major retail stores such as Loews, Home Depot, or Menards. If you don t have samples you can use, do some research into the types of woods and then answer the questions. On your worksheet, please answer the following questions: Which one would you use for a deck or floor? Which one would you use if you were making something like bookshelves? Which one is denser? Which one do you think is probably more expensive? Which one do you think is more fire resistant? Which one do you think is used for making paper?

4 Old Giants While gymnosperms number less than 1,500 species and comprise less than five percent of the whole plant kingdom, they win the prize for verified longevity. The Great Basin Bristlecone Pine of Nevada is estimated to be nearly 5,000 years old and the Giant Sequoia of California is estimated to be over 3,000 years old. These are just two of the oldest trees, but all of the top five are gymnosperms. I find it interesting that these trees would have been among the first to grow after Noah s flood. It is no coincidence that these trees do not rely on animals to spread their seeds as the angiosperms do. They rely on wind, which there was plenty of at that time, while animals were pretty scarce. That is one possible reason why these trees grew first and still survive. Another is that gymnosperms in general are faster-growing and more resilient than deciduous trees when faced with adverse conditions. They also produce many more seeds than their flowering counterparts, which we will explore next. Cones are not for ice cream Do you sneeze a lot in early spring from environmental allergies? It is likely that tree pollen is making you miserable. Gymnosperms produce two types of cones: male and female. The male cones produce pollen in the spring, and it is dispersed by the wind to hopefully land on female cones. These are usually near the top of the tree or near the outer branch, but this is not true for all species. The male cones are often small, herbaceous, and inconspicuous, while the female cones are the ones we all see, being an identifying feature of different species. Just as with other plants, once the pollen finds the female ovule, it develops a pollen tube to deliver the male gamete, forming a seed. This process can take a few years. After the seeds are formed, the tree drops its cones. However, this is influenced by the conditions in

5 which the tree lives. Some trees will not drop their cones until the soil is dry. Some seeds will not sprout until triggered by tremendous heat, as in forest fire. These measures ensure that the seeds have the best chance of survival and that forests will grow back (in fact, in some areas, controlled burns are the normal way to encourage this new growth).