Plant Organs: Roots, Stems, and Leaves PDF

Organs and Organ Systems Vegetabilia have three different types of body construction The most primitive pla...

Organs and Organ Systems Vegetabilia have three different types of body construction The most primitive plants have thallus body, more advanced is the shoot (unipolar) plant body, and most land plants have the bipolar plant body. The Plant Organs: Roots, Stems, The thallus plant body is flat, similar to leaf but do not differentiated into particular and Leaves organs. Most gametophytes have this type, and also few sporophytes (which mostly are reduced water plants). Shoot (unipolar) plant body consists only of branching shoots, roots are absent. This is typical to all Bryophyta sporophytes, mosses (Bryopsida) gametophytes, and also to sporophytes of Psilotopsida (whisk ferns). Finally, bipolar plant body has both shoots and roots. Most bipolar plants have shoots consist of stems and leaves, but this is not an absolute requirement since young plant stems are normally green2and can do photosynthesis. e d c a b Ulva lactuca Sargassum sp. Seagrass Figure 5.8. Evolution of plants body types: a–e thallus gametophytes, (a) thallus sporophyte, (b–d) shoot sporophyte, (e) bipolar sporophyte. 1  hypocotyl is a part of stem between first leaves of the seedling (cotyledons) and root (i.e., stem/root transition place), epicotyl epicotyl is first internode of stem cotyledons hypocotyl Cotyledon or seed is a chimeric structure with three genotypes so it is impossible to call it “organ”. Root, stem, leaf and FU (Floral Units) are four basic plant organs (Fig. 5.10) which in bipolar plant could be grouped in root and shoot system; Figure 5.9. Young seedling with epicotyl and hypocotyl. the latter is frequently split into generative shoot system (bearing FU), and vegetative shoot system6 (without FU). Typical organs of bipolar plant are floral unit (FU) terminal (apical) bud stem pedicel generative shoot system stems (axial aerial organs with continuous growth), node leaves (flat lateral organ with restricted growth), internode roots (axial soil organ modified for absorption) and axial (lateral) bud terminal bud on secondary shoot vegetative s hoot system floral units (FU) which are elements of the generative system secondary shoot (fructifications) such as a pine cone or any flower. leaf blade petiole adventitious roots Buds, fruits, seeds and specific to seedlings hypocotyl and epicotyl hypocotyl main root (taproot) are non-organs for different reasons: lateral roots root system buds are just young shoots, fruit is the ripe flower, 7 Figure 5.11. Systems of organs and organs of bipolar plant. 2  terminal flower The Leaf buds The first and ultimate goal of every plant is photosynthesis. If a plant is multicellular, it usually develops relatively large, flat structures which goal is to catch sun rays. axillary leaf scars buds Terrestrial plants are no exception; most probably, they started to build their body with organs similar to present day leaves. easy to detach A leaf is lateral photosynthetic organ of shoot with restricted growth. Its functions are photosynthesis, respiration, transpiration, and Figure 5.12. How to distinguish compound leaves (left) from synthesis of secondary chemicals. branches (right). Features of a leaf (i.e., characters help to distinguish it) include having a bud Morphology of the Leaf There are three types of leaf characters: general, terminal, and repetitive. Morphology means external, well visible structural features whereas Anatomy needs tools like a microscope and/or scalpel. General characters are only applicable to the whole leaf. Leaves are very important in plant morphology. The ability to describe the leaf Terminal characters are only applicable to the terminal leaflets. is a must even for novices in botany. Terminals are the end parts of leaves, they do not split in smaller terminals; clover leaf, for example, has 3 terminals. Simple leaves have just one level of hierarchy whereas compound leaves have two or more levels of hierarchy. Lastly, repetitive characters repeat on each level of leaf hierarchy. Compound leaves are sometimes mixed with branches but there are many General and terminal characters do not depend on hierarchy. other characteristics which allow to distinguish them Repetitive characters may be different on each step of hierarchy. 3  Repetitive characters are the shape of the leaf, leaf dissection, and whether the blade is stalked (has petiole) or not. Terminal characters are applicable only to terminal leaflets of leaves. These characters are the shape of the leaf blade base, the leaf tip, the type of margin, the surface, and the venation. 1 level 2 levels 3 levels The base of the leaf blade could be rounded, truncate (straight), Figure 5.15. Leaves with one, two and three levels of hierarchy. Please note that the cuneate, and cordate. last leaf is ovate on the first and second level but circular on the third level of hierarchy. The leaf apex could be rounded, mucronate, acute, obtuse, and General characters of leaf include stipules and other structures located near leaf acuminate. base, the sheath (typical for grasses) and ocrea (typical for buckwheat family, Polygonaceae). Leaf margin variants are entire (smooth) and toothed: dentate, serrate, double serrate and crenate. Figure 5.17. Leaf shapes. Stipule is the proper word to describe the small, leaf-like structures that grow at the base of a leaf stalk. These structures can vary in shape and size, and may be present on one or both sides of the leaf. Ocrea, on the other hand, is a term used to describe a sheath-like structure that surrounds the stem of a plant. This structure is typically found at the base of a leaf, and can be used to protect the plant from insects and other potential threats. 4  Figure 5.19. Figure 5.18. Base of leaf blade Terminal leaf Leaf characters dissection. rounded straight cuneate cordate Leaf tip rounded mucronate acute obtuse acuminate Leaf margin entire dentate serrate double-serrate crenate Lateral/ Main No One Several Leaf veins are vascular bundles coming to the leaf from stem. Frequently, there is a main vein and lateral veins (veins of second Apodromous Hyphodromous Acrodromous order). There are multiple classifications of leaf venation; No Note that in dichotomous venation, each vein divides into two similar parts which is known as dichotomous branching. The example of dichotomous venation is the leaf of maiden hair tree, ginkgo (Ginkgo biloba). Dichotomous Pterodromous Actinodromous Several Another frequently segregated type of venation is parallellodromous, but in essence, this is acrodromous venation in linear leaves (for example, leaves of grasses) where most of veins are almost parallel. The simple classification of leaf venation 5 To characterize the whole leaf, one might use the following plan:. To characterize the whole leaf, one might use the following plan: 1. General characters (leaf as a whole): (a) stipules (present / absent, deciduous / not, how many, size, shape); (b) base (sheath / no sheath, ocrea / no ocrea Terminal characters (leaflets): (a) base of leaf blade (rounded, truncate, cuneate, cordate) 2. First level of hierarchy: repetitive characters: (b) apex (rounded, mucronate, acute, obtuse, acuminate); a. symmetry (symmetrical / asymmetrical); (c) margin (whole, dentate, serrate, double serrate, crenate ); (a) shape; (d) surfaces (color, hairs etc.); (b) dissection; (e) venation (apo-, hypho-, acro-, ptero-, actinodromous) (c) petiole (presence and length) 3. Second level of hierarchy 4. Third level of hierarchy, and so on 5. Terminal characters (leaflets): Heterophylly refers to a plant having more than one kind of leaf. Modifications of the leaf include A plant can have both juvenile leaves and adult leaves, water leaves and air leaves, or sun leaves and shade leaves. spines or scales for defense, tendrils for support, A leaf mosaics refers to the distribution of leaves in a single plane perpendicular to light rays, traps, “sticky tapes”, or urns for interactions (in that case, catching insects), this provides the least amount of shading for each leaf. plantlets for expansion, and succulent leaves for storage. Leaves have seasonal lives; they arise from the SAM through leaf primordia, and grow via marginal meristems. The old leaves separate from the plant with an abscission zone. Plantlets are little mini plants that grow on the main plant and then fall off and grow into new plants; the most known example is Kalanchoë (“mother of thousands”) which frequently *** uses plantlets to reproduce. The famous poet and writer Johann Wolfgang Goethe is also considered a founder of plant Plants that have insect traps of various kinds are called car- nivorous plants (in fact, they are morphology. still photoautotrophs and use insect bodied only as fertilizer). He is invented an idea of a “primordial plant” which he called “Urpflanze” where all organs Several types of these are the cobra lily (Darlingtonia), various pitcher plants (Nepenthes, were modifications of several primordial ones. In accordance to Goethe’s ideas, plant Cephalotus, Sarracenia), the butterwort (Utricularia), the sundew (Drosera), and the best 23 24 morphology considers that many visible plant parts are just modifications of basic plant known, the Venus flytrap (Dionaea). organs. 6  Anatomy of the Leaf Anatomically, leaves consist of epidermis with stomata, cuticle mesophyll (kind of parenchyma) and vascular bundles, or veins upper epidermis (Fig. 5.22). palisade mesophyll xylem vascular bundle (vein) The mesophyll, in turn, has palisade and spongy variants. spongy lower phloem Palisade mesophyll is located in the upper layer and serves to decrease epidermis cuticle the intensity of sunlight for the spongy mesophyll, and also catches slanted sun rays. The palisade mesophyll consists of long, thin, tightly arranged cells with chloroplasts mostly along the sides. The spongy mesophyll cells are roughly packed, they are rounded and 25 26 have multiple chloroplasts (Fig. 5.21). The Stem Leaves will also reflect adaptations to the substrate, ecological forms named The stem is an axial organ of shoot. It has functions of support, transportation, psammophytes (grow on sand), photosynthesis, and storage. Stem has radial structure, no root hairs and grows petrophytes (grow on rocks), and continuously. rheophytes (grow in fast springs). The latter plants frequently have serious simplifications in their body plan, their leaves and stems are often reduced to Morphology of the Stem form a thallus-like body. Stem morphology is simple. Its components are nodes (places where leaves are/were attached) and internodes, long or short (in the last case, plant sometimes Parasitic plants could be classified in mycoparasites, hemiparasites, and appears to be stemless, rosette-like). phytoparasites. Stems are different by the type of phyllotaxis. Mycoparasitic plants feed on soil fungi, phytoparasitic plants are either The phyllotaxis refers to the arrangement of leaves. plant root parasites or plant stem parasites lacking chlorophyll and If there is one leaf per node, it is a spiral (alternate) arrangement. photosynthesis. Two leaves per node means opposite arrangement. Opposite leaves can be all in the same plane or each pair can rotate at 90◦. Hemiparasitic plants are those which still have chloroplasts but take the If there are more than two leaves per node, it is a whorled arrangement, and each signifi- cant part of water and even organic compounds from the host plant whorl can also rotate. (like mistletoe, Viscum). Each type of spiral phyllotaxis has its own angle of divergence. Multiple types of spiral leaf arrangement mostly follow the Fibonacci sequence: 7  Anatomy of the Primary Stem Plant evolution resulted first in the primary stems with no lateral meristems and secondary tissues. Only long after plants “learned” how to thicken their stems. a b c d protoderm Figure 5.24. Types of phyllotaxis (leaf SAM arrangement): a spiral (alternate), b and c ground meristem opposite, d whorled. pith epidermis This sequence of numbers made with simple rule: in the every following fraction, time the numerator and denominator are sums of two previous numerators and denominators, respectively. The sequence looks fairly theoretical but amazingly, it is fully applicable to plant science, namely to different types of spiral phyllotaxis phloem phloem xylem xylem Figure 5.26. Developmental origin of stem tissues (simplified). Letters e, p, a show respec- tively where endoderm, pericycle and vascular cambium might appear. Development of stem starts from stem apical meristem (SAM) on the top of plant. The SAM produces three primary meristems: procambium, protoderm, and ground The outer layers of the procambium form the primary phloem. The inner layers become the primary xylem. Epidermis with cuticle Parenchyma between bundles Exoderm The middle layer can be entirely spent or will make cambium for the secondary Cortex parenchyma thickening. Pith (or central hole) At times, the layers of the outside of the procambium can form a pericycle. Endoderm Sometimes the innermost layer of the cortex can form an endodermis (endoderm) Procambium and outermost layer makes the exodermis (exoderm). Xylem All these layers are some kind of the “border control” between functionally different Cambium layers of stem. Another frequent variant is the development of collenchyma in the Phloem Vascular cortex adjacent to epidermis. bundle Pericycle Vascular bundles connect leaves and stems. In many plants, they form a ring on the Anatomy of the primary stem (right). Slanted font is used for cross-section of the stem. Parenchyma (ground tissue) between vascular bundles “optional” tissues. Small image on the left is the young stem typically belongs to both cortex and pith. consisted of epidermis, cortex, procambium and pith. Another variant is a vascular cylinder, 31 structure which fully encircles the stem. 32 Liliid (monocot) stems generally have dispersed vascular bundles. 8  Roots employ many different modifications which help to protect, interact and storage. The Root For example, roots of parasitic plants are modified into haustoria which sink themselves into Root is a latest evolutionary innovation in the vegetative plant anatomy. Many the vascular tissue of a host plant and live off of the host plant’s water and nutrients. “primitive” plants (all mosses and even some ferns like Psilotum) do not have roots; some flowering water plants like the rootless duckweed (Wolffia) or the Roots of mangroves (plants growing in ocean coastal swamps) are frequently modified into supportive aerial roots (“legs”). coontail (Ceratophyllum) have also reduced their roots. However, large Since these swamp plants need oxygen to allow cell respiration in underground parts, there are homoiohydric plants need the constant supply of water and minerals, and this pneumatophores, special- ized roots which grow upward (!) and passively catch the air via evolutionary challenge was responded with appearance of the root system. multiple pores. Plants which grow on sand (psammophytes, see above) have another problem: their substrate Root in an axial organ of plant with geotropic growth. One of root functions is constantly disappears. To avoid this, plants developed contractile roots which may shorten to supply anchorage of the plant body in soil or on various surfaces. Other and pull plant body deeper into the sand. functions include water and mineral absorption and transport, food storage, Some orchid roots are green and photosynthetic. However, as a rule, root is the heterotrophic and communica- tion with other plants. organ, because root cells have no access to the light. Root nodules present on the roots of nitrogen-fixing plants, they contain bacteria capable to 1. Morphology of the Root deoxidize athmospheric nitrogen into ammonia: N2 → NH3. There are two types of root systems. The first is a fibrous root system which has multiple big roots that branch and form a dense mass which does not Root nodules contain also hemoglobin-like proteins which facilitate nitrogen fixation by keeping have a visible 33 oxygen concentration low. Nitrogen-fixing plants are especially frequent among faboid rosids: legumes (Leguminosae family) and many other genera (like alder, Alnus, or Shepherdia, buffaloberry) have root nodules with bacteria. Anatomy of the Root On the longitudinal section of young growing root, there are different horizontal layers, zones: root cap covering division zone, elongation zone, absorption zone, and maturation zone. The root cap protects the root apical meristem (RAM), which is a group of small regularly shaped cells. A small, centrally located part of the RAM is the quiescent center where initial cells divide and produce all other cells of root. Root cap is responsible for the geotropic growth, if the root tip comes into contact with a barrier, root cap will feel it and will grow on a different direction to go around it. The elongation zone is where the cells start to elongate, giving it length. The ab- sorption zone is where the rhizodermis tissue (root hairs) develops and where water and nutrients are absorbed and brought into the plant. Figure 5.32. Root zones: 0 root cap, 1 division zone, 2 elongation zone, 3 absorption zone, 4 maturation zone. Within the maturation zone, root hairs degrade, many cells start to acquire secondary walls and lateral roots develop 9 On the cross-section of the root made within absorption zone, the first tissue is the rhizodermis, which is also known as the root epidermis, then cortex, which segre- gates external exodermis and internal endodermis one-cellular layers, and vascular cylinder. Typically, roots have no pith. In some cases (for example, in orchids), cortex may give multi-layered velamen (see above), another absorption tissue. Vascular cylinder is located in the center of the root, it contains the pericycle which is made of mostly parenchyma and bordering endodermis. Pericycle cells may be used for storage, they contribute to the vascular cambium, and initiate the development of lateral roots. Consequently, lateral roots are developing endogenously and break tissues located outside, like aliens in the famous movie. Figure 5.33. Left to right, top to bottom: Ranunculus root with 4-rayed xylem, Salix root with the Root phloem is arranged in several strands whereas xylem typically has a radial, lateral root developing, Smilax root with visible Casparian stripes in the endodermis; Zea root sometimes star-shaped struc- 37 longitudinal section with root cap, division and elongation zones. Magnifications ×100 (second) and ×40 (others). Water and Sugar Transportation in Plants Plants need water to supply photosynthesis (the oxygen is from water!), to Vascular Xylem Endodermis cool down via transpiration, and to utilize diluted microelements. Dead cylinder Phloem Parenchyma Cortex velamen (paper-like), rhizoids (hair-like), and living rhizodermis (rhizoderm) Cambium Exodermis are responsible for water up- take. Pericycle Rhizodermis In rhizodermis, root hairs increase the surface area where the plant has to absorb the nutrients and water. To take water, hair cells increase concentration of organic chemicals (the process which needs ATP) and then use osmosis. There are two ways that water transport may go: apoplastic or Figure 5.34. Anatomy of root: cross-section through the maturation zone. symplastic. Apoplastic transport moves water through the cell walls of cortex: from the rhizodermis to the endodermis. Endodermis cell walls bear Root tissues develop in the way similar to stem, RAM gave rise to ground meristem, Casparian strips (rich of hydrophobic suberin and lignin) which prevent the procambium, and the protoderm, which in turn make all primary tissues mentioned water from passing through the cell wall and force symplastic transport (Fig above. Later, pericycle develops into lateral roots or the vascular cambium which in 5.35) through cytoplasms and plasmodesmata. Symplastic transport there is turn produces into the secondary xylem and phloem. The secondary root is similar to directed to the center of root only and requires ATP to be spend. secondary stem (see below). By pumping water inside vascular cylinder and not letting it back, endodermis cells create the root pressure 40(Fig. 5.36). It is easy to observe on tall herbaceous plants 10  Root pressure Casparian strip The apoplastic route Root hair The symplastic route Figure 5.35. Symplastic and apoplastic transport in root. 41 Figure 5.36. The origin of root pressure: water comes into vascular cylinder but cannot go back because of endoderm (brown line). The only possible way is to go up. 11