WHMF121 Session Seven Plant Morphology Stems

Transcription

1 WHMF121 Session Seven Plant Morphology Stems Endeavour College of Natural Health endeavour.edu.au 1

2 Todays Topic o Plant stems: Stem development Vascular bundles Modified stems Function of stems Endeavour College of Natural Health endeavour.edu.au 2

3 Stems o When a seed germinates the radicle grows downwards and develops into the roots, and the plumule grows upwards and develops into the stem. We often may think of stems as fragile structures, however, think upon the fact that the trunk of this magnificent redwood is a stem, that would take an entire football team hand to hand to encircle. This specimen is circa 180m high and almost 2000 years old. ( Endeavour College of Natural Health endeavour.edu.au 3

4 o Development: Dicotyledon Stems Epigeous or Epigeal - e.g. garden beans video The cotyledons may be carried above the ground during germination as the stem grows Hypogeous or Hypogeal - e.g. pea video The cotyledons remain underground within the seed coat and the stem grows up from them. (Refer to Session 3 - Germination) In both cases the cotyledons provide nourishment for the plant until it grows foliage leaves that can photosynthesise. Endeavour College of Natural Health endeavour.edu.au 4

5 Monocotyledon Stems o Development: Epigeous or Epigeal - e.g. onion Allium cepa. The cotyledon elongates, carrying the seed coat up with it and the foliage leaves develop near the base. Hypogeous or Hypogeal - e.g. corn Zea mays The seed coat and the first foliage leaves emerge directly from the seed surrounded by the coleoptile. Video of corn developing a stem Permission given from Neil Bromhall Endeavour College of Natural Health endeavour.edu.au 5

6 Monocots: Vascular Bundles o Vascular bundles are present and scattered throughout stem, although concentrated towards the outside. Dicots: o Vascular bundles differ as a they form in a ring towards to outer part of the stem - often none in the centre. o This is true to some extent of almost all monocots. Monocots rarely produce secondary growth and are therefore seldom woody, with Palms and Bamboo being the exceptions. Endeavour College of Natural Health endeavour.edu.au 6

7 Stems Roystonea regia (Royal palm) (Aracaceae family) ( Phyllostachys bambusoides (Bamboo) (Poaceae family) ( _Phyllostachis_bambusoides.JPG) Endeavour College of Natural Health endeavour.edu.au 7

8 Stems are needed for transporting plant requirements Phloem Pushes the sugars and other molecules created by the plant through photosynthesis down to roots Phloem is always alive Xylem Pulls moisture up from the roots This tissue dies after one year and then develops anew (rings in the tree trunk) Becomes the wood Endeavour College of Natural Health endeavour.edu.au 8

9 Stem Cross-Section of a Dicot Flax stem crosssection, showing locations of underlying tissues Ep = epidermis C = cortex BF = bast fibres P = phloem X = xylem Pi = pith ( Endeavour College of Natural Health endeavour.edu.au 9

10 Zea mays stem (left) and root (right) cross sections Observe the difference in the arrangement of the vascular bundles ( Endeavour College of Natural Health endeavour.edu.au 10

11 Stems o The terms stems and shoots are sometimes used interchangeably but it must be remembered that the stem is the axis. (This will be especially relevant when looking at flowers). o Stems have nodes. o The shoot is the stem from which any buds, new stems, leaves and flowers emerge from. o All flowering plants have the same basic stem structure, whether they are dicots or monocots. (Mauseth, 2014) Endeavour College of Natural Health endeavour.edu.au 11

12 Stems o Stems grow in a series of nodes and internodes o At the nodes, the stem tends to widen a bit and have the appearance of a joint o Leaves grow at the nodes of a stem o The angle between the stem and the leaf is called the leaf axil ( nt_nodes_c.jpg) Endeavour College of Natural Health endeavour.edu.au 12

13 Stems o Lateral stems or branches may also grow at a node, from the axillary bud (in the leaf axil) o If anything were to happen to the leaf, the axillary bud would start to develop Texas A&M Bioinformatics Working Group 2010 ( Endeavour College of Natural Health endeavour.edu.au 13

14 Stems o Nodes are a point of proliferation o On some stems, like rhizomes and stolons, roots as well as stems grow at nodes giving rise to a new plant o Internodes are the areas of stem between the nodes ( Endeavour College of Natural Health endeavour.edu.au 14

15 Stems Node of a Monocot ( Endeavour College of Natural Health endeavour.edu.au 15

16 Stems o Like roots, stems do most of their growing near the tip. o New leaves and branches start to develop in minute form in the bud - the tip of a stem or shoot. o As the tip grows the leaves develop and mature. o Two aspects to stem growth: Development of structures at the nodes Elongation of the internodes. o At this growing tip the nodes are developing directly one after another. Endeavour College of Natural Health endeavour.edu.au 16

17 Stems o The stem tip has an inhibiting effect on lateral stem growth. o This is called apical dominance Cutting off the tops of plants makes them grow bushier. o The inhibiting effect is decreased the further the growing tip is from a node. o Therefore as the main stem grows, lower nodes are able to develop lateral stems or branches. ( wiki/apical_dominance) Endeavour College of Natural Health endeavour.edu.au 17

18 Stems A hedge is an excellent example of apical dominance. As the plant stems grow up towards the light, trimming them promotes lateral growth and makes the plant bushier. Hedges are often used as decorative ornamental features in a garden, but also as wind breaks for farmers crops. ( Endeavour College of Natural Health endeavour.edu.au 18

19 Stems o Most plants depend on light for nourishment o One of the functions of stems is to hold the leaves up to the light o Like roots, stems have a mechanism for directing their growth o Root growth is directed by gravity (geotropism) o Stem growth is directed by light (phototropism) ( Endeavour College of Natural Health endeavour.edu.au 19

20 Stems othe tips of the stems contain the hormone auxin which regulates plant growth by causing cells to grow longer. oauxin is involved in phototropism. oif a stem begins to grow away from the light, light will fall unevenly on the stem Auxin migrates to the darker side of the stem, so the cells will lengthen at a faster rate than those on the light side and the stem tip will eventually point upwards. Permission granted by CSIRO to show video link? Endeavour College of Natural Health endeavour.edu.au 20

21 Types of Modified Stems The modified stems fall into categories: Aerial stems - Tendrils - Thorns - Cladodes Sub-aerial stems - Stolon - Runners Underground stems - Bulbs - Corms - Rhizomes - Tubers Endeavour College of Natural Health endeavour.edu.au 21

22 Stolon Stolons are modified lateral stems that grow across the ground and can send out roots and new shoots. Many grasses and other plants like potatoes, violets and strawberries have stolon. Stolons enable plants to perform vegetative or asexual reproduction, i.e. offspring are formed without the production and union of sex cells. Offspring from asexual reproduction are genetically identical to the parent plant. A common example is Fragaria spp. (strawberry). Endeavour College of Natural Health endeavour.edu.au 22

23 Rhizomes o Rhizome comes from the Greek meaning mass of roots o Rhizomes are modified underground stems. o Like stolons they are very common in grasses and occur in many other plants. o They vary in size and appearance. o Some plants have thick, fleshy rhizomes like Zingiber officinale (ginger) but others like Urtica spp (nettle) and Symphytum officinale (comfrey) do not. Endeavour College of Natural Health endeavour.edu.au 23

24 Rhizomes o Each node can send out roots and stems and develop into a whole new plant. o Rhizomes may look like roots since they grow underground o Roots - do not have nodes with leaves or axillary buds and lateral stems. o Rhizomes do have these structures though they often look different to those of a typical stem. ( r/file:zingiber_officinale_fresh_rhizome.jpg) Endeavour College of Natural Health endeavour.edu.au 24

25 Tubers o Tubers are underground stems especially adapted for the storage of starch. o A well known tuber is the potato (Solanum tuberosum). It grows from the end of either a stolon or a rhizome of the potato plant. o The eyes of potatoes are the axils of tiny, scale-like leaves and contain axillary buds from which lateral stems may grow. o Root tubers also exist but they do not have leaves or buds since roots do not have nodes. Endeavour College of Natural Health endeavour.edu.au 25

26 Bulbs o Bulbs consist of a small central stem surrounded by modified, fleshy leaves. o The leaves are scaly above and thickened at the base where food is stored. o They are often found just under the earth and adventitious roots grow from the bottom of the stem. o Bulbs serve as storage and propagative organs. o After the growing season the foliage leaves wither and die. Endeavour College of Natural Health endeavour.edu.au 26

27 Bulbs o The energy from them is drawn down into the bulb and stored through the winter to be used when the plant grows again. o Plants in the Amaryllidaceae family and related families often have bulbs e.g. Allium cepa (onion) and Allium sativum (garlic). ( Endeavour College of Natural Health endeavour.edu.au 27

28 Corms o Corms are like bulbs except the storage organ is the fleshy stem which is surrounded by thin scales, the bases of last season s harvest. o Like bulbs, corms are found just under the ground and have adventitious roots growing from the base of the stem. ( Endeavour College of Natural Health endeavour.edu.au 28

29 Corms Colchicum autumnale Autumn Crocus Once a corm has flowered it never flowers again and starts to develop new corms for next season as the old corm starts to die off ( ale_enbla05.jpeg) Endeavour College of Natural Health endeavour.edu.au 29

30 Spot the Difference? Corm Solid mass of stem tissue Bulb The inner leaves are fleshy while the outer ones are dry Endeavour College of Natural 30 Health endeavour.edu.au 30

31 Cladodes o In most plants photosynthesis is performed by the leaves, but in some the stem has evolved to be the photosynthetic organ. o These stems are green because they contain chlorophyll and are called cladodes. o Equisteum arvense (horsetail) and Ephedra sinica (ma huang) are medicinal cladodes o. o Casuarina spp. (she-oaks) and Opuntia spp. (prickly pear) are also plants with cladodes. Endeavour College of Natural Health endeavour.edu.au 31

32 Aerial modifications Cladodes o The leaves of Opuntia spp. have evolved to be thorns or spines o Ephedra sinica & Equisteum arvense - the leaves are like scales at the nodes of the stem. Equisteum arvense (Photograph with permission - Wendy Williams) Endeavour College of Natural Health endeavour.edu.au 32

33 Cladodes Opuntia spp. (Prickly Pear) ( Endeavour College of Natural Health endeavour.edu.au 33

34 Functions of Stems 1 - Support Stems hold up the aerial parts of the plant and orient the leaves towards light. Tendrils are sometimes modified stems. Passiflora spp. (passionfruit vines) have tendrils at the ends of lateral stems that twist around whatever they encounter and hold the plant up. Many plants employ tendrils to provide added support. Of the herbal medicines we use, Passiflora incarnata is an example of one that utilises tendrils. ( Endeavour College of Natural Health endeavour.edu.au 34

35 2 - Conduction Functions of Stems o Like roots, stems contain vascular tissues that transport fluids around the plant body. o Remember the two kinds of vascular tissue, xylem and phloem, they run next to each other all through the plant. o When water is absorbed into the roots it is passed to the xylem and drawn up through the roots and stems by evaporation of moisture from the leaves. o This fluid reaches the leaves and is enriched by the glucose produced during photosynthesis. o This enriched sap is then carried around the plant by the phloem. Endeavour College of Natural Health endeavour.edu.au 35

36 Observing Conduction with Food Dye (Image Credit: ECNH Lecturer, Tanya Quod, 2013) Endeavour College of Natural Health endeavour.edu.au 36

37 Stems A cross section of Apium graveoluns (Celery) Apiaceae ( Endeavour College of Natural Health endeavour.edu.au 37

38 Functions of Stems o The xylem typically lies close to the axis with phloem away from the axis. o The xylem is closer to the centre of the stem or root while the phloem is closer to the exterior. o In a leaf, the xylem will usually be the upper side, with the phloem on the lower side. o This is why aphids are typically found on the underside of a leaf rather than on the top - the sugars manufactured by the plant are transported by the phloem, which is closer to the lower surface. o The position of vascular bundles relative to each other may vary considerably Endeavour College of Natural Health endeavour.edu.au 38

39 Aphids Under Leaf ( ( ds_and_live_young_under_sycamore_leaf.j PG) Endeavour College of Natural Health endeavour.edu.au 39

40 Tree Trunk Cross Section ( Tree Music Endeavour College of Natural Health endeavour.edu.au 40

41 Functions of Stems 3. Storage o In many plants energy produced by the leaves during warmer months is stored over winter in modified stems (e.g. bulbs, corns, tubers, rhizomes). o This allows the plant to become dormant over the colder months. o When spring returns this energy is used to grow new leaves and roots. Endeavour College of Natural Health endeavour.edu.au 41

42 Functions of Stems o Storage in these stems is different to storage in roots because the energy is stored to allow dormancy than simply to be used later. o Other storage stems include fleshy or bulbous stems like celery, and succulent stems that can store water. (Image Credit: ECNH Lecturer, Tanya Quod, 2013) Endeavour College of Natural Health endeavour.edu.au 42

43 The Boab prime storage area! Adansonia gregorii ( Endeavour College of Natural Health endeavour.edu.au 43

44 4. Propagation Functions of Stems o Most stem modifications seem to have evolved for this purpose. o Rhizomes and stolons can grow new plants at the nodes, and bulbs and divide over winter so more plants are produced in spring. o Tubers store a lot of starch and can grow new roots and shoots from the eyes. Endeavour College of Natural Health endeavour.edu.au 44

45 Propagation Propagation by stem cuttings Endeavour College of Natural Health endeavour.edu.au 45

46 Tutorial Session o Practical: Check on your monocotyledon and dicotyledon seeds germinating in the cottonwool and notice the changes since last week o Participate in the class exercises (following slides) regarding stems Endeavour College of Natural Health endeavour.edu.au 46

47 Practical Session o Draw a germinating plant. Label cotyledons, roots, stem. o Draw a typical stem and label the nodes, internodes, leaf axil and axillary bud. o Draw a cross section of a celery stalk. Label xylem. Endeavour College of Natural Health endeavour.edu.au 47

48 Practical Session Draw a stolon. Label main stem, stolon, nodes and internodes. Refer to p. 123 of text book (Capon) Endeavour College of Natural Health endeavour.edu.au 48

49 Practical Session Draw Zingiber off. (Ginger) Rhizome Label the nodes, internodes, axillary buds and scale like leaves (Image Credit: ECNH Lecturer, Tanya Quod, 2013) Endeavour College of Natural Health endeavour.edu.au 49

50 Practical Session Draw Solanum tuberosum L. (Potato) tuber Label the attachment scar, node, leaf, leaf axil, axillary bud ( e:potato_sprouts.jpg) Endeavour College of Natural Health endeavour.edu.au 50

51 Practical Session Cut Allium cepa (onion) in half lengthwise and draw bulb Label roots, stem and leaves ( Endeavour College of Natural Health endeavour.edu.au 51

52 Next Week o Preparation: Read through the slides for session 9 on leaves Read the section in your textbook on leaves Bring some bunches of leaves if you have access to them so we can look more closely at them and draw some diagrams Endeavour College of Natural Health endeavour.edu.au 52

53 Suggested Readings Capon, B. (2010). Botany for gardeners (pp ). Portland, OR: Timber Press. Clarke, I., & Lee, H. (1987). Name that flower: The identification of flowering plants (p. 41). Carlton, Vic: Melbourne University Press. Mauseth, J. (2014). Botany: An introduction to plant biology (5 th ed.). MASS: Jones & Bartlett Publishers. Tan, E. (2013). Botany of the flowering plants (4 th ed.). (pp ). Preston, Vic: Northern Melbourne Institute of TAFE. Endeavour College of Natural Health endeavour.edu.au 53

54 References Clarke, I., & Lee, H. (1987). Name that flower: The identification of flowering plants. Carlton, Vic: Melbourne University Press. Tan, E. (2004). Herbal preparations laboratory manual. Victoria, Australia: Northern Melbourne Institute of TAFE. Tan, E. (2013). Botany of the flowering plants (4 th ed.). Preston, Vic: Northern Melbourne Institute of TAFE. Wohlmuth, H. (1992). An introduction to botany and plant identification (2 nd ed.). Lismore, NSW: MacPlatypus Productions. Endeavour College of Natural Health endeavour.edu.au 54

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