Lecture 10 - Stems and Leaves

seed plants vascular (1) monocot?

*Eustele* - separate bundles of xylem and phloem *monocot* - vascular bundles scattered throughout; thought to have evolved from eustele (cortex or pith not evident)

modified stems

*Rhizomes* - horizontal stems that grow below ground and have long to short internodes. *Tubers* - Underground swollen ends of rhizomes that store food (potato) *Stolons* - modified stems that run above the soil surface (strawberry) *Water-storage* - Desert cacti have succulent stems *Thorns* - modified stems that protect plant from herbivores

SVPs vascular (2)

*protostele* - similar anatomy as roots *Siphonostele* - continuous vascular cylinder that surrounds a pith; evolved from protostele

storage tubers corms bulbs

- *Tubers* are underground stems that *store food* - *Corms* are stems that *store nutrients* - *Bulbs* are *short stems* with *fleshy leaves* attached

whats a stem

- A stem and its leaves are commonly known as a shoot - *Move leaves* toward light and away from shade - *Support* the weight of leaves - Withstand wind - *Conduct* water, minerals and organic molecules between roots and leaves

leaf formation

- Arise from the shoot apical meristem (*SAM*) - Initiate as a bulge (*leaf buttress*) that is round - Leaf *primordia* flanking the meristem grow and eventually flatten, giving rise to the *petiole and blade*

vascular tissue 3 types of stele

- Considerable variation exists in the arrangement of vascular tissue (*stele*) in primary stems - A few seedless vascular plants have the simplest type of stele [protostele] (similar to that seen in roots) - Fossil record indicate that *protostele* were present in the earliest plants found *Stele *- Central cylinder of primary xylem, primary phloem, and pith (if present) - *Protosteles* - Solid core, phloem surrounds xylem (in primitive seed plants, ferns, club mosses and whisk ferns) - *Siphonosteles* - Tubular with pith in center (Common in ferns) - *Eusteles* - Discrete vascular bundles (in flowering plants and conifers)

first plant leaves? innovative feature? do they all look the same?

- First plant leaves were *photosynthetic stem systems* (branched, flattened, and eventually developed lamina) - Leaves are biological *"Solar panels"* with automatic tracking and other innovative features - Leaves come in *many shapes and sizes*

abscission zones

- From the Latin word meaning *"to cut off"* - Deciduous plants (trees) occur in temperate as well as tropical regions (regulated by seasonal changes in water status) - *Protective layer* of cells forms that *prevents pathogens* for gaining access - shedding of leaves

growth of stems? SAM? intercalary meristems

- Grow in *length* but also *produce* leaves and axillary branches - Stems lengthen simultaneously at several internodes below the growing point at the shoot apical meristem (*SAM*) - Some plants have *intercalary* meristems (dividing cells at each internode) allowing *rapid growth* along the length of the stem

leaf veins netted vs parallel

- Leaf veins are continuous with the vascular bundles in the petiole and stem - Veins form under the influence of the plant hormone *auxin* - The *upper* portion of the vein usually consists of *xylem* and the *lower* portion of *phloem* - There are two general patterns of veins: netted and parallel *netted* - occur in dicots and ferns *parallel* - occur in monocots and gymnosperms

nodes, internodes, and axillary buds

- Leaves are attached to stems at *nodes* - The *internode* is the space between leaf attachment points - Dormant *axillary buds* that can potentially form branches are located between the stem and the upper leaf petiole

what determines phyllotaxy (2 theories)

- Phyllotaxy is determined when leaves initiate 1. *Field theory* (Biochemical theory) - Inhibitory *chemical* signal is produced by a growing *primordium* 2. *Available-space theory* (Biophysical Theory) - Forces from *existing primordia* cause a *spontaneous* buckling of the apical surface that gives rise to the *next primordium*

leafs as classification

- Plant identification guides use leaf traits for classification

phyllotaxy - leaf order (3)

- Stems have variations in leaf arrangement - Each species of plant has a predictable pattern to this arrangement - There are three basic patterns: 1. *alternate* - 1 leaf per node (aka spiraled in primitive flowers) 2. *opposite* - 2 leaves per node 3. *whorled* - 3 or more leaves per node (seen in floral organs)

asexual reproduction stolons rhizomes

- Strawberry stolons/ runners (horizontal aboveground stems) originate as axillary buds - Iris rhizomes are horizontal underground stems

connecting root and shoot

- Transitions from protostele of root to vascular bundles in stem - Occurs within *short distances* and occurs early in seedling development - *Early plants* did not have roots but underground stems (rhizomes) with protostele (roots retained this configuration)

structural support

- Transitions from protosteles to siphonosteles and eusteles may have evolved to provide better structural support

specialized functions (7)

1. *Drought tolerance*: xerophytes with sunken stomata and thick cuticles 2. *Protective leaves*: spines (thorns are modified stems and prickles are modified epidermal cells) 3. *Tendrils*: attach plant to supporting structure 4. *Flotation*: extra air pockets (Water Lilies) 5. *Bracts*: Colourful foliage (Poinsettias) 6. *Asexual reproduction* 7. *Insect*-eating leaves to acquire nitrogen

tissue types that make up the leaf

1. *Epidermal* tissues - provide *protection and gas exchange* - Hairs control *moisture loss and predation* - The *cuticle* and surface waxes (epicuticular waxes) provide *protection* from water loss, light irradiation and pathogen infestation - The *epicuticular* layer also serves as a *semipermeable* barrier and regulates the passage of molecules into and out of the plant (endogenous and exogenous) - *Regulate* gas exchange and transpiration (evaporation of water) - Specialized epidermal cells, called *guard cells*, regulate gas exchange through openings called stomata 2. *Mesophyll* tissue - (Greek mesos, "middle", phyllon, "leaf"), the ground tissue, carries out *photosynthesis* - 2 types: a) *Palisade mesophyll* are elongated cells usually located immediately *below the upper epidermis* (*tightly packed*) b) *Spongy mesophyll* cells are located *below this layer* and are *loosely packed* and have sufficient air spaces to allow *CO2 diffusion* - contain *chloroplasts* and are specialized for photosynthesis 3. *Vascular* tissues - form *veins* (with different patterns) that *transport* organic molecules, water and minerals between leaves and the rest of the plant - provide *support*

stem variations in palms vs wheat

palms: - Grow in tropical areas - *Perennial* - Apical meristem protected from most animals (*off the ground*) - *Height* improves access to light - All internodes elongate wheat: - Grow in temperate zones - *Annuals* - Shoot apical meristem *near the ground* (protected from the elements and from grazing animals) - Elongates to produce flowering head - Intercalary meristems - Tillers (axillary shoots)