Life and Diversity III: Plants Dr. Waggoner BIOL 1400 A multicellular protist such as Coleochaete, shown here highly magnified, is made up of many cells. But most of the cells are the same this is a protist, not a true plant. On the other hand, this magnified cross-section through a leaf shows a larger number of cell types cells of different shapes, sizes, positions, and functions. So what s a plant? Almost all plants are photosynthetic Chlorophyll is the green pigment that is used to capture solar energy Exceptions: a few parasitic plants, such as "Indian pipe" Cell walls made of cellulose When this is mature, it prevents a plant cell from expanding in size Many functioning plant cells are dead the cell wall is all that s left Growth at the tips of roots and shoots, in meristems Special zones of immature cells without thick cell walls. Cells in meristems can expand, divide and grow. 1
2/9/11 Dodder (Cuscuta spp.), the yellow tangle seen here, is an exception to the rule that plants are photosynthetic. Dodder penetrates into host plants and robs nutrients directly from them, potentially killing the host plant! But the vast majority of plants are autotrophs and carry out photosynthesis running their metabolism on solar energy, water, carbon dioxide gas, and nutrients taken from the soil. Indian paintbrush (Castilleja coccinea) is another parasitic plant. Although it does photosynthesize, its roots invade the roots of nearby plants (usually grasses) and draw food and water from them. To do this, plants use a substance called chlorophyll (actually, they use a mixture of two slightly different varieties of chlorophyll) that can absorb light energy and convert it into chemical energy. 2
This chemical energy is used to convert carbon dioxide gas (CO 2 ) and water (H 2 O) into the simple sugar glucose (C 6 H 12 O 6 ), which can be used to build more complex sugars (like those found in maple syrup), to build other molecules, or to fuel the plant s metabolism. Many plants contain other pigments yellow xanthophylls, orange carotenoids, and red and blue anthocyanins. These may not be visible in the plant until the chlorophyll disappears in the fall. In fact, in many plant tissues, like this cork layer from the outer bark of an oak, the cells die when the tissue matures only the cell walls remain behind. Obviously, such cells can't grow. As you ve seen, mature plant cells are surrounded by a cell wall made of a carbohydrate called cellulose (unlike fungi and bacteria). This limits a mature plant cell s ability to expand and grow. 3
This longitudinal section through a plant stem shows a dense cap of cells at the very tip. This is the apical meristem, the only place where cells are dividing to make the main stem longer. Lateral meristems along the sides will grow into branches. So what s a plant? Waxy layer, or cuticle, surrounds the outer layer of cells Typically there are several specialized organs: Roots absorb water and nutrients; may be used to store nutrients (as in beets) Stems usually support; may photosynthesize. Leaves are usually the main site of photosynthesis. Modified leaves include vine tendrils, cactus spines. TIP: If it has buds on it, it s not a root, it s a stem. Potatoes aren t roots, they re tubers underground stems that are modified for food storage. Bulbs (such as onions, tulip bulbs, iris bulbs) are also modified stems (usually surrounded by thick leaves) not roots. Underground, horizontal stems are known as rhizomes as in crabgrass, Johnson grass, and many more. 4
2/9/11 Plant classification is complex, but for our purposes we can divide plants into two main types. Bryophytes, like this moss, are nonvascular meaning that they lack specialized tissues that transport water. With no water-transporting tissues, bryophytes have limitations on where they can grow. In this section through an alfalfa stem, the two types of conducting tissue are visible: xylem, for water, and phloem, for sap. Tracheophytes, a group that includes almost all the plants that you re familiar with, are said to be vascular plants. Translation: They have specialized cells that transport water and nutrients throughout the plant. Quiz 3 1. Name two places where you might find extremophile archaeans. 2. Name two features that eukaryote cells have but prokaryote cells lack. 3. Name two examples of beneficial bacteria. 4. Eukaryotes that don t have cell differentiation get lumped into the Kingdom. 5. If I told you to find me a mycelium, where would you look and what would you look for? 5