Plant structure

VI. Plant Structure

A. Classification of Plants (see pp. 391 - 405)

1. Some plants are non-vascular (=bryophytes)
2. Some plants are seedless, vascular plants (e.g., ferns)
3. Most plants are seed-bearing, vascular plants
- gymnosperms (no flowers, e.g., conifers)
- angiosperms (all produce flowers: dicots and monocots)
- monocots reproduce by "double fertilization" [Fig. 25.14], their leaves have parallel veins, and they have vascular bundles distributed throughout the ground tissue of the stem (palms, lilies, orchids and grasses -- including major food crops)

B. Structure of Flowering Plants (= angiosperms)

1. Overview

a. Shoots and roots [Fig. 29.2]
- shoots = above ground parts -- stems provide support for leaves (photosynth.), flowers, fruits
- roots = parts under the ground -- absorb water and dissolved nutrients
b. Three tissue systems
- ground tissue -- bulk of plant body
- vascular tissue -- distribution of water and solutes
- dermal tissue -- covering, protection of plant surface
c. Meristems = local regions of embryonic (undifferentiated) cells
- apical meristems -- tips of stems and root where growth results in increase in length of stems and roots; produced by cell division and enlargement
- lateral meristems -- growth produces thickening of stems and roots; vascular cambium produces vascular tissues, cork cambium produces cork
- meristem cells can divide; mature, differentiated cells are specialize in structure and function and usually do not divide. Continued divisions of meristem cells keeps a plant growing throughout it's life = indeterminate growth. Differentiated daughter cells form relatively permanent parts of the plant.
[If you tie a swing on a tree branch does it move farther from the ground each year? What happens if you tie a ribbon around the trunk of a young tree?]
d. Primary and secondary growth [Fig. 29.4]
- primary growth: as a stem (or root) grows longer, primary meristems develop behind the growing tip; these differentiate into the three tissue systems:
protoderm --> epidermis (waxy covering in shoots)
procambium --> vascular cambium
--> primary xylem and phloem
ground meristem -->ground tissues
--> cork cambium
- secondary growth: in woody plants vascular cambium forms a ring; each growing season a new layer of xylem is produced which pushes the cambium and all outer tissues further out. Old phloem cells are crushed and only a thin strip of newly- formed phloem remains alive. [Fig. 29.20]
- as the stem expands, it ruptures the epidermis; cork cambium differentiates from parenchyma cells in the cortex and divides to form cork. Multiple layers of dead cork cells replace the epidermis and protect the stem
- not all plants show secondary growth: monocots and some dicots have little or none; they are non-woody plants. Many dicots that are woody and all gynmosperms show secondary growth during two or more growing seasons.

2. Simple tissues [Fig. 29.5]
- parenchyma -- most abundant; thin-walled, pliable cells; alive at maturity; retain capacity for cell division. Various types: photosynthesis, storage, secretion.
- collenchyma -- thickened cell walls that help strengthen plant; often arranged in strands or cylinders beneath dermal tissue (e.g., "strings" in celery)
- sclerenchyma -- thick, secondary cell walls for mechanical support and protection in mature plants. Often impregnated w/ lignin (strong, waterproof).
3. Complex tissues
- vascular: xylem, phloem. [Fig. 29.6]
xylem conducts water, minerals abs. from soil; provides mechanical support. Cells (tracheids, vessel members) are dead at maturity.
phloem transports sugars, other solutes. Cells (sieve tube members) are alive at maturity.
- dermal tissues: epidermis, later periderm [Fig. 29.7]
waxy cuticle on surface restricts water loss; specialized epidermal cells (= guard cells) form stomata --> regulate movement of water vapor, carbon dioxide, oxygen across epidermis

C. Monocots vs. Dicots [Fig. 29.8]

1. Monocots include grasses, lilies, irises, palms; dicots include common trees and shrubs (other than conifers = gymnosperms)
2. Monocot seeds have one cotyledon (seed leaf); dicot seeds have two.
3. Monocots have vascular bundles distributed throughout ground tissue of stem; dicots have bundles of vascular tissue positioned in a ring in stem. [Fig. 29.9a and 29.9b -- What's wrong with these overheads?]. Xylem faces center; vascular ring separates ground into pith and cortex in dicots.
4. Monocot leaves have parallel veins and the base of the leaf blade encircles and sheathes the stem; dicots leaves have netlike veins and the flat portion (blade) is usually connected to the stem by a stalk (petiole).
5. Monocot flowers typically have parts that occur in threes or groups of threes; dicot floral parts occur in four or fives or multiples or fours or fives.

D. Leaves

1. Layers (top to bottom) [Fig. 29.12]:
cuticle --> upper epidermis --> palisade mesophyll --> spongy mesophyll --> lower epidermis (contains stomata) --> cuticle
2. Leaf structure/function
- photosynth. in parenchyma cells in mesophyll
- vein = vascular bundles for movement of water, solutes, products of photosynth.
- air spaces for gas exchange
- stomata (mostly on lower epidermis) to regulate water loss
3. Origin of leaves [Fig. 29.13]
- leaves develop on the flanks of the tips of stems as outgrowths of apical meristems
- buds are meristem tissues covered by modified leaves; terminal buds occur at the tips of shoots; lateral buds form in the upper angles where the leaf petiole is attached to the stem.

E. Roots

1. Functions
- absorption of water, minerals
- anchor plants in the soil
- store food produced by photosynth. in plant parts above
2. Internal structure
- cells in apical meristem divide, then differentiate into root epidermis, ground tissues and vascular tissues
- root epidermis has extensions = root hairs for absorptive interface w/ environment
- vascular tissues form a vascular cylinder arranged as a central column surrounded by cortex; endodermis = innermost layer of cortex surrounds vascular cylinder and helps control water movement into it [Fig. 29.17]
- pericycle lies just inside endodermis; = meristematic tissue that can give rise to lateral roots [Fig. 29.19]