BIOL 153L General Biology II Lab Black Hills State University Lab 10: Plant Roots, Stems, & Leaves Structurally, angiosperms are characterized by having roots and shoots, the latter including stems, leaves, and flowers. Although the aforementioned plant parts have characteristic functions and forms, each may be modified to cope with environmental challenges and take advantage of resource opportunities. Today's lab will focus on vegetative (non-reproductive) parts of angiosperms with some reference to corresponding parts of gymnosperms. Note that angiosperms are often divided into two groups, monocots and dicots. Although molecular data suggest the distinction between these groups is more complicated than once thought, there are nonetheless characteristic differences in the roots and shoots of monocots vs. dicots that will be referenced in this course. Roots (Chapter 24) lack leaves and nodes. They are generally the descending part of the plant found beneath the soil surface. However, some roots are aerial (above the soil) and/or adventitious (arising from stems or leaves). The functions of roots are anchorage, absorption, and/or storage. Shoots (Chapter 25) are the above-ground part of the plant and include stems, leaves, and flowers structurally very different parts that have distinctive functions. Stems are best known as the ascending portion of plants, upon which leaves and flowers attach. However, some stems occur underground, such as rhizomes or corms. The primary functions of stems are transport of water and nutrients, elevation of leaves and flowers, and storage. Leaves are generally lateral appendages that connect to the stem at the node. They are extremely variable in shape and the main location of chloroplasts and stomata. The primary functions of leaves are food production, gas exchange, and cooling via transpiration. Flowers are reproductive structures, composed of four floral whorls sepals, petals, stamens, and pistils. The primary function of flowers is sexual reproduction, as discussed in Lab #9. Roots Taproots are central, dominant roots and characteristic of dicots. The first structure to emerge from most seeds is the primary root, which may give rise to lateral roots. Oldest lateral roots are near the stem. Fibrous roots are thin and branching and characteristic of monocots. The primary root is often short-lived in monocots, and the adult root system arises from stem nodes. Fibrous roots can grow from rhizomes, which are underground stems (Figs. 24-2 & 24-3, pp. 560-561). 1. Examine taproot systems on display and then sketch below. On the taproot system, label primary root and lateral roots. 1
2. Examine fibrous roots and then sketch below. 3. Rhizomes are underground stems and often superficially look like roots. Rhizomes enable plants to spread clonally in some plant species, a large 'patch' or 'clump' represents one genetic individual that is connected belowground! Examine rhizomes and sketch examples below. 4. Plant growth occurs from meristem tissue ('plant stem cells'). All cells in a given plant contain the same DNA, but most are differentiated for particular functions and patterns of gene expression (e.g., petal cells cannot ordinarily develop into root hairs). However, meristem has undifferentiated cells that can grow into a variety of plant tissues and organs. Thus, adventitious roots can arise from the meristems of non-root structures (e.g., stems or in a few cases, leaves). a. In ivy (Hedera, Araliaceae), adventitious roots develop at nodes the area where leaves attach to the stem. Locate the display of ivy plants that has been set up by your lab instructor. Examine and sketch adventitious roots and associated stem of English Ivy (Hedera). English Ivy is a groundcover, and under natural conditions, one individual plant may sprawl across hundreds of square feet. Following germination, a young ivy seedling has a tap root. At maturity, however, adventitious roots are very abundant on ivy stems. Why do you think adventitious roots are important to English Ivy growth? 2
b. In Pregnant Plant (Kalanchoe, Crassulaceae), small plantlets with adventitious roots develop on the edges of adult leaves. Locate the Pregnant Plant display set up by your lab instructor. Examine and sketch an entire Pregnant Plant with plantlets. Take a plantlet from the labeled petri plate, and using a dissecting scope, view the roots and look for protrusions from the root surface. Draw a magnified view of a plantlet and roots below. How do Pregnant Plant plantlets help disperse the species? Are the plantlets genetically identical to or different from the 'parent plant' that makes them? Explain. 5. Examine the large plastic root model on display in the lab. Roots have three primary areas of development. The region of cell division, or the apical meristem, lies behind the root cap and is where active cell division occurs. The region of elongation is where cells increase in length and drive the root tip through the soil. The region of maturation is where root cells differentiate. Root hairs arise from the epidermis in the region of maturation and while, single celled and short lived, greatly increase root surface area. Regions of cell division and elongation are only a few centimeters in length; there are not visible boundaries between them. (Chapter 24, Figs. 24-8, 24-9, pp. 564-565). On the plastic model, locate and then sketch the root cap, region of cell division (apical meristem), region of elongation, region of maturation, and root hairs. 3
6. Examine prepared slide 242 (root hairs wm). [Note: also examining prepared slide 243 (Zea root tip) is optional but encouraged, it offers a better root cap view and is useful for understanding root growth in general.] Sketch the root tip in the space below, and label the root cap, region of cell division (apical meristem), region of elongation, region of maturation, and root hairs. a. What is the function of the root cap? Why does it occur before the apical meristem? b. Even young plants have billions of root hairs that, in aggregate, cover an enormous surface area! Compare the structure/function of root hairs vs. lateral roots using the table below. (Check the box of the root type that is described by each trait.) Trait Root Hair Lateral Roots Composed of one cell Composed of multiple cells Extensions of epidermis Primarily for absorption Transports water and nutrients Short lived (days) Long lived (weeks or years) 7. Symbiotic relationships with roots. The roots of many plants are colonized by mycorrhizal fungi. The fungal mat can extend hundreds of meters, and the mycorrhizae may provide nutrients (especially phosphorous) to plants. In return, the plants provide sugars to the fungus. There is even evidence that fungal mats connect multiple plants in a population, and through the fungi, individual plants may share the sugars they produce with other individuals in the population. Some plant species, most notably legumes, are colonized by nitrogen-fixing bacteria that reside in root nodules. The bacteria receive a protected place to live, and in return, plants are rewarded with nitrogen. Legumes are sometimes called green manure because they add nitrogen back to the soil. This link discusses how trees share resources and communicate with each other via mycorrhizae. Do Trees Communicate, https://www.youtube.com/watch?v=isgpnm3bfmq 4
Questions a. According to the video, what resources do trees share with one another? b. What is a mother tree? (Notice the example is a Douglas Fir this is the species from lab 8 that has mouse tail shaped sterile bracts on cones.) c. What organism connects these trees to one another? d. How can dying trees help the remaining living trees? How can timber harvest harm the surviving trees? Leaves 1. Examine fresh leaves on live plants. Much of the leaf is composed of the blade, which is the major region of photosynthesis and transpiration. Most leaves attach to the stem via the stalk-like petiole (some leaves lack petioles). Buds form at the base of petioles. Veins are generally visible on the leaf blade; the central, prominent vein is called the midrib, while prominent veins are visible across the blade (Fig. 25-17, p. 591). Dicots typically have net veins (reticulated and web-like) and monocots have parallel veins (Figs. 25-19b & 25-25, pp. 592 & 598). Examine and sketch the leaves on display. Label the blade, petiole, midrib, and veins (net or parallel). Include the species name and indicate whether the plant is a monocot or dicot. 5
What is the purpose of leaf veins? 2. Examine leaf morphology on herbarium sheets. Simple leaves have blades that are not divided into distinct parts, though they can be lobed. Compound leaves are divided into many distinct leaflets at first glance a compound leaf may appear to be many small leaves but instead it is one larger leaves composed of distinct leaflets. To identify compound leaves, look for the lateral buds. Leaves have buds at their base (where the petiole connects to the stem). In a compound leaf, the bud will not be at the base of the leaflets (Fig. 25-20, p. 593). Examine angiosperm leaf collage. Sketch leaves in boxes below. Simple leaves (1 lobed, 1 unlobed) Pinnately compound leaf Palmately compound leaf lobed unlobed Next, sketch leaves and stems from herbarium sheets showing 'simple' vs. 'compound' leaves. Label petiole, bud, 1 leaf, and leaflets (if compound). Plant names are on sheets! White Poplar, Populus alba (simple) Boxelder, Acer negundo (compound) 6
3. Examine leaf arrangement ('phyllotaxy') on herbarium sheets. Leaves can attach to a stem directly opposite one another; in this case, leaf arrangement is opposite. Leave attachment can also alternate on the stem; in this case, leaf arrangement is alternate (Chapter 25, pp. 590-592). Next, sketch leaves and stems from herbarium sheets showing 'alternate' vs. 'opposite' leaf arrangements. Label petiole, bud, and leaflets (if compound). Plant names are on sheets! White Poplar, Populus alba (alternate, simple) Boxelder, Acer negundo (opposite, pinnately compound) Red-osier Dogwood, Cornus sericea (opposite, simple) Woodbine, Parthenocissus vitacea (alternate, palmately compound) Now test yourself! Determine whether the five unknowns set up in the lab have simple or compound leaves, and opposite or alternate leaves record your answers on the table shown on the next page. (Species names and specimen numbers are recorded on herbarium sheets.) 7
1. Cottonwood 2. Black Walnut 3. Silver Maple 4. Green Ash 5. Siberian Elm Simple or compound? Alternate or opposite? 4. Examine winter branches. Leaves form inside buds and are protected by bud scales. Lateral (=axillary) buds are found along the stem at the nodes, where petioles attach. Terminal buds are found at the ends of branches (Figs. 25-1 &26-16, pp. 580 & 624). Buds are compressed, embryonic shoots that contain apical meristem, stem and immature leaves and flowers. Plants only grow longer (primary growth) from apical meristem thus, all stem elongation starts with a bud. Buds remain dormant until conditions are suitable for growth however, while buds may not elongate when dormant, they may be meristematically active. Leaves and flowers can do much of their development while protected by bud scales. This allows them to quickly expand when conditions are suitable and the buds open (Fig. 25-6, p. 583; pp. 676-678). a. Sketch the branches on display there are several species, so be sure to include the names with your drawings! Label nodes, terminal buds, bud scales, and lateral buds. State whether leaf arrangement is opposite or alternate. b. What is primary growth? Where will it occur on the branch? 8
5. Examine highly-modified leaves. Some plants have leaves that are modified for functions besides photosynthesis. Spines of cacti are an example! Sketch cactus, showing spines (modified leaves) on the surface of the 'pad' (modified stem). 6. Many plants have sharp structures extending from their surfaces. However, these sharp appendages originate from different parts of the plant. As mentioned, spines are modified leaves. In contrast, thorns and prickles are modifications of the stem and epidermis! Examine the thorny branch put on display by your instructor. Thorns are modified stems, often stout, and sometimes have buds or leaves growing from them. Sketch the branch showing thorns. Indicate the species. Examine the prickly branch put on display by your instructor. Prickles are extensions of the epidermis, and are often very abundant and flexible (but still painful to touch!). Sketch branch showing prickles. Indicate the species. Why do plants invest energy into making sharp appendages like prickles, thorns and spines? 9
7. Examine prepared slide 32 (Populus leaf cs). When looking at the leaf cross section, imagine how the slice was made through the leaf. The sides curve toward the center and at the center is the midrib. The upper and lower epidermis are the outer layers of the leaf. The long palisade cells, which are generally rich with chloroplasts, lie beneath the upper epidermis. The spongy layer, which tends to have few chloroplasts and lots of air space, lies between the palisade cells and the lower epidermis. Stomata (guard cells and pores) occur in the epidermis. Many times, they are only found on the lower epidermis. (Chapter 25, Fig. 25-21, p. 594). Sketch the Populus leaf cross section. Label upper epidermis, lower epidermis, palisade cells, spongy layer, stomata, guard cells, pore, chloroplasts, air space, and veins/vascular tissue. a. Why does the palisade layer have more chloroplasts than the spongy layer? b. Why might guard cells be more common on the lower epidermis? c. Why is there air space in the spongy layer? 8. Examine prepared slide 87 (monocot and dicot epidermis wm). Previously, you examined Hedera leaf peels to count and measure stomata. This slide is similar. However, rather than being a leaf impression molded from the surface, this is actual epidermis peeled from the leaf. Epidermis protects the plant from water loss (cuticle) and regulates gas exchange (stomata). Stomata are composed of lip-shaped guard cells and the central pore. Sometimes, stomata are surrounded by subsidiary cells. Stomata tend to be haphazardly arranged in dicots and linearly arranged in monocots. The non-specialized cells are called epidermal, or pavement cells. Epidermal cells tend to be longer and narrower in monocots than in dicots. 10
Sketch the monocot and dicot epidermal peels. Label stomate, guard cells, pore, subsidiary cells, epidermal/pavement cells. Be sure to show how stomatal arrangement differs. Dicot leaf peel Monocot leaf peel 9. Examine prepared slide 56 (pine leaf two-needled). Plants that grow in very dry places will sometimes have sunken stomata and waxy/tough foliage to reduce water loss. Some angiosperms show this feature, as do most conifers. Pine needles, which are pine leaves, are retained on the tree for several years, and many of them must survive multiple cold and dry winters. They also have resin ducts, in which produce sticky pine pitch (= sap). (Chapter 18, Fig. 18-14, p. 438). Sketch the pine needle cross section. Label sunken stomata, resin ducts, epidermis, and veins. How does resin production help the pine tree? (Brainstorm, and think about pine forests!) Seed germination When seeds germinate, the primary root (= radicle) is usually the first structure to emerge. It grows downward. The hypocotyl, which give rise to the shoot, then grows upward. Cotyledons (aka seed leaves) open when exposed to light. Cotyledons will provide energy to the young plant until true leaves develop, and they will often remain attached to young seedlings for quite a while. Some seed leaves look leafy, but will be a different shape than true leaves. Other times they are fleshy and rounded. Dicots have two cotyledons while monocots have one cotyledon (see textbook, Chapter 22, Figs. 22-9 & 22-10, pp. 534-535). 11
1. In your textbook, carefully inspect Figure 22-9 (p. 534) and 22-10 (p. 535) and then sketch a generalized drawing of a germinating seed in the space below. Be sure to label the major parts, including primary root (radicle), root hairs, hypocotyl, cotyledons, seed coat, and true leaves. 2. Next, watch this time-lapse video of a germinating bean seed and answer questions below. Lima Bean Timelapse: http://y2u.be/izmjbo6a7ae a. What is the first part that emerges from the germinating bean seed? What does this become? b. How does the hypocotyl emerge from the soil? (Describe or draw your response.) How does this emergence strategy protect the vulnerable parts of the young plant? c. What do the cotyledons (i.e. seed leaves) look like in a bean? Is this species a dicot or monocot? d. What do the first true leaves look like in a bean? (Describe or draw your response.) e. Are the radicle and hypocotyl sporophyte (2x) or gametophyte (1x) tissue? 12
Trait Summary for Monocots vs. Dicots: Over several labs in the second half of the semester, we introduce characteristics to distinguish monocots from dicots. Fill in the table below with distinguishing features covered in today s lab. Monocot Dicot Leaf venation (net or parallel) Number of cotyledons (one or two) Arrangement of stomata (linear or haphazard) Root type (taproot or fibrous) 13