LECTURE - Lecture 2 + 3
Form and Function
Plant Life-Cycle Duration
1. Herbaceous plants
Herbaceous plants do not have extensive woody tissues
As a result, after they end their life cycle, they decompose rapidly
Based on their life duration, herbaceous plants can be:
Annual - plants live for a single growing season during which they germinate, grow vegetatively, form flowers, and produce fruits and seeds
Biennial - live for two years:
Plants are vegetative in the 1st year and then produce flowers/fruit/seeds in the 2nd year
Perennial - live for three or more years and usually flower and fruit yearly once plants reach sexual maturity
How do you Recognize Annual Versus Perennial Plants
Annual plants do NOT have modified underground stems (right)
Perennial plants DO have modified underground stems; e.g., rhizomes, tubers, bulbs, etc. (left)
Perennial herbaceous plants usually survive from one year to the next via underground, modified stems
Even if their above ground parts are destroyed in the late fall-winter, buds from the underground stems will regenerate the plants in the next spring
You do not need to know any of the examples; just understand what the terms above mean and be able to recognize them
Practical Application: A Visit to the Garden Center
Most garden centers have distinct sections for annuals and perennials
Which one to buy?
Simple:
If you want something just for the summer of that particular year, buy annuals
If you re planning a long-term garden, also invest into perennials
Annuals flower abundantly, literally “to death” because they have only a few months to produce flowers and seeds
Perennials are more “restrained” because they distribute their reproductive assurance across multiple years
Also, usually perennial plants have some means of vegetative reproduction (e.g., via underground stems) in case the sexual reproduction (via seeds) fails
2. Woody plants: are always perennial
Woody plants possess extensive woody tissues that are added annually, and which take a lot of time to decompose after they die
Woody plants are always perennial
Shrubs = have multiple stems from the ground
Trees = have mainly one single stem – “the trunk” or main stem
The plant in the image is:
Annual
Biennial
Perennial
Young trees
Young shrubs
This is…
A tree
A shrub
A herbaceous plant
A biennial plant
Extremes
Ephemeral plants:
Live for 2-6 weeks
Can have sometimes 2-3 life cycles every year
E.g., Arabidopsis thaliana
Extremely long-living plants
Thousands of years
E.g., Giant sequoia
This is a huge variation in the duration of life cycle of plants
Tissues - Classification
Tissue:
A group of similar cells that carry out a specific function
Organs are formed by the functional grouping and connection of multiple tissues
Meristems produce ALL the other (“definitve”) tissues
1. Meristems
Meristems produce all the other tissues, and therefore, they are responsible for plant growth and development
Meristem cells divide mitotically and newly resulted cells differentiate into specific tissues
Meristematic cells are the least differentiated cells in the body of a plant; they are small and densely packed together, with no intercellular spaces
Their cell wall is thin, cellulosic
They have dense protoplasm; their vacuoles are very small; they lack differentiated plastids (e.g., chloroplasts, chromoplasts)
Meristem Classifications
There are several types of meristems and → you have two simple, but important classifications
We say about a definitive tissue that it has a primary origin when it is produced by primary meristems or a secondary origin when it’s produced by secondary meristems
Apical Meristems
Apical meristems generate growth in length
They are found at the growth tips of the plant: e.g., of roots and shoots, and consist of primordial and primary meristems
Lateral Meristems
Lateral meristems generate growth in thickness and they are always secondary meristems
There are 2 types of lateral meristems:
Vascular cambium
Cork cambium
They both form the dedifferentiation of primary cell tissues (which regain their mitotic division ability)
Vascular cambium produces secondary xylem toward the interior and secondary phloem at the exterior [“interior” = towards the center of the stem; "exterior" = toward the exterior of the stem]
Cork cambium produces the periderm (bark)
Which of the following meristems develops first in the body of a growing plant?
Procambium
Primordial
Fundamental
Cork cambium
Protoderm
With So Many Meristems/Mitosis, Why Don’t Plants Develop Cancer?
Something that sets apart plants and animals… although trillions of mitotic divisions take place in the meristems of each plant, these are so well coordinated and controlled, tumors do not normally develop in plants
Plant tumours are very rare and usually contained because it much more difficult for cells to escape tissue/organism controls
Applications of Meristems: Plant Tissue Culture (in vitro propagation)
Meristem cells are totipotent
Totipotency = is the ability of a single cell to divide (mitotically), differentiate and reproduce the entire organism
Meristem cells are totipotent
Explant the plant part with meristems that will be used to generate callus
Callus = growing mass of cells resulted from the explant cells that divide mitotically
In living plants, callus also develops, for example, to help the healing of a wound
Most commonly used explants are the apical shoot and parts or leaves, but the explant can be even a single cell
In plant tissue culture (or micropropagation), callus formation is induced in sterile conditions by plating explants onto culture media within a closed (sterile) vial
The culture medium is supplemented with nutrients and plant growth regulators (also called plant hormones) which will guide the regeneration of the plant
The essence of in vitro micropropagation is the possibility of obtaining thousands or even millions of plants from just an explant
The only downside is that all the plants resulted will be clones, thus lacking genetic diversity
Callus Examples
Callus looks weird, but using plant hormones one can control the development of the plant from it
Biotechnology Applications of Plant Micropropagation
The applications below prove how powerful plant micropropagation is for biotechnology
1) In vitro micropropagation allows growing massive amounts of plants starting from very little material
Many cultivated plants are propagated these days in vitro; ornamental plants (e.g., orchids) but also potatoes, strawberries, etc.
Micropropagation has also applications in plant conservation
Downside - reduced genetic variation
2) Plant genetics and breeding:
E.g., callus to radiations and select desired mutants
Or create a transgenic cell and then regenerate an entire plant from it (containing the transene in all its cells) from it
3) Production of secondary compounds tissues grown in vitro; thus, there is no need to grow the entire plant to extract those compounds; just its callus
4) Cytological research (e.g., study of mitosis)
2. Parenchyma (Ground Tissue)
Parenchyma is most common and abundant tissue in a plant - that’s why it is also called “ground tissue”
It is present in all the plant organs
Consists of living cells with thin, cellulosic walls
It has various roles; e.g., storage of nutrients, water, air; photosynthesis, etc., depending on the position within the plant and the ecology of that particular plant
I has either primary or secondary origin depending on the type of meristems they originate from
3. Collenchyma [Mechanical Tissue]
Mechanical tissues ensure the plant's capacity to withstand such static and dynamic mechanical stresses such as the force of gravity and gusts of wind
Collechyma has support and flexibility function; it provides elasticity (e.g., in such a way herbaceous stems or leaf petioles do not break when the wind blows
It is a living tissue that originates from parenchyma cells in which the primary walls become thickened with additional layers of cellulose
In our example, the supplementary deposition of cellulose took place at the angles between the cells (angular collenchyma)
Often found in the ribs of stems and leaf petioles, immediately under the epidermis
Collenchyma is usually a primary tissue
4. Sclerenchyma
Sclerenchyma has also a mechanical, resistance function
In some cases, it may also provide elasticity like the collenchyma (e.g., fibers in the leaves and stems of grasses), but most often it confers physical hardness/toughness, like the skeleton of animals
It is made up from dead cells because the primary cell walls become very thickened with lignin
The lignified cell walls fill nearly all the cell’s volume
Can have primary or secondary origin and it is much more specialized and diverse than collenchyma
Classification of sclerenchyma:
A) Sclereids: cell length and width are more or less equal
B) Fibers: cell length exceeds MANY times the width (fibers cells are very long)
4.1. Sclereids
Sclereids are amazingly variable in shape; cells can be isodiametric, rectangular, branched or star-like
But compared to fibres, sclereids are short (length and width are more or less the same)
Characteristic examples are the "stone" cells (called this way because of their hardness) found in the fleshy part of pear fruits, giving them a gritty texture
The seed coat of many seeds and the frut wall, as well as the “stones” of cherries, plums, peaches, olives - are made up from sclereids of different shapes
Aquatic plants have often benched sclereids (“astrosclereids”) that develop strategically to reinforce aerenchyma intercellular spaces in the stems and leaves
4.2. Fibers
Fibers are more uniform, very long cells and they can be present in the roots, stems, leaves, seed coasts and fruit walls
People have learned to separate the process textile fibers to make fabric thousands of years ago
Uses: ca. 40 plant sp. produce textile fibers extracted from stems or leaves
Cotton
Hemp (Cannabis
Flax
Yucca
Stinging nettle
This tissue in the image is:
Storage parenchyma
Aerenchyma
Chlorechyma
Collenchyma
Sclerenchyma
Meristem
Vascular Tissues
5. Xylem (“Wood”): Tracheart Elements (SEM View)
Xylem conducts “raw sap” - the water and minerals absorbed by the roots to the leaves
Xylem conductive elements = tacheary elements are dead, elongated cells with cell wall thickened with lignin
Although dead, they are still capable to fulfill their role of conducting water and minerals
Lignified cell walls of xylem elements are transferred by numerous pits, which ensure the fluid circulation among cells
Because cell walls of xylem are very thick and hard, xylem has also a mechanical role providing rigidity (think of woody plants)
Tracheary Elements - Optical Microscope
Staining with toluidine blue, which colors lignin in green-blue
Tracheary Elements - Types
There are two types of xylem conductive (tracheary) elements:
Thacheids are longer and thinner
Vessel members are shorter and wider
Both form long, tube-like structures that are present in the entire body of the plant
But the xylem - especially the secondary xylem (wood) produced by the vascular cambium in angiosperms is a very complex tissue that in addition to tracheids and/or vessel members also includes other types of cells:
Parenchyma
Cells
Rays
Fibers
No need to recognize these elements in a structure, just to realize that not all the wood is “dead
Its parenchyma cells are alive
6. Phloem
Phloem transports sugars and other organic assimilates resulted from the photosynthesis from the leaves to the rest of the tissues in the plant
Sieve cells form long tubes and are connected to one another through sieve plates
Although sieve tubes are living cells, their nucleus disintegrates and they require the companion cells to function
The transport via phloem is "active" and it requires ATP
Like xylem, phloem can also have a primary or secondary origin
Phloem as a tissue - especially the secondary phloem - may also be accompanied by sclerenchyma
Which one of these is tracheids?
A
B
D
7. Epidermis
The epidermis is characteristic to young organs, with primary structure
In roots and stems of eudicots, it is replaced by the periderm when the secondary structure develops
Epidermis of Roots
Root epidermis has absorbent hairs and it is usually 1-cell layered
The main role of epidermis in roots is absorption of water and minerals
Epidermis of roots has a thin or no cuticle
Epidermis of Above Ground Organs
Main role in above ground organs (e.g., stems and leaves) is protection and usually the epidermis possess a distinct cuticle
Often, the epidermis cells on leaves have irregular outlines and interlock with one other like jigsaw puzzle-pieces to form a sturdy layer
Characteristic to the epidermis of above ground organs is the presence of hairs (trichommes) and stomata
Stomata are involved in the gas exchange and transpiration
They can be present in the epidermis of all the above ground organs (e.g., stems, leaves, even flowers) but they are characteristic to the leaves
A stoma (plural stomata) consists from:
Guard cells
Accessory (subsidiary) cells
Guard cells can close or open forming a pore through which the gas exchange takes place
The cells surrounding the guard cells are called accessory or subsidiary cells
What are the arrows pointing at?
Accessory (subsidiary) cells
Companion cells
Guard cells of stoma
Parenchymatic cells of stoma
None of the above
Non-Secretory Trichomes
We call them non-secretory because they don’t secrete anything (see "Secretory cells and tissues” next)
Non-secretory trichomes are very diverse morphologically and can have many roles:
Reduce transpiration/evaporation
Mechanical role (e.g., deter herbivores)
Reflect UW light
Retain water
Attach seeds or fruits to animal fur as a mean of dispersal (of course, when present on seed or fruit epidermis)
How many different tissues do you see in this cross-section through a stem?
1
2
3
4
5
8. Periderm
Like epidermis, periderm is a protection tissue and it becomes the outermost tissue of roots and stems with secondary structure, most commonly in woody plants
In such plants, the periderm replaces the epidermis completely
Periderm is always produced by the secondary tissue called cork cambium
The cork cambium produces on its outer side layers of cork cells impregnated with suberin which are dead at maturity, and on the inner side produces phelloderm, a living, parenchyma tissue
The periderm includes all these tissues:
The cork cambium and its products
In temperate climates, the activity of cork cambium is seasonal and every year, a new periderm will develop
However, each new periderm usually develops at the interior of the previous ones, therefore, the outer layers begin to break and exfoliate in patterns that are characteristic for each species
This is especially evident in trees and we call these successive exfoliating periderms the “bark”
Many trees can be easily recognized by their bark
9. Secretory Cells and Tissues
Secretions have a bewildering chemical diversity and their role is not always well-understood
Often secondary metabolites secreted by plants have medicinal value
Flavour and fragrance of plants also are the product of secretion
Cells or groups of living cells (tissues) that produce a huge variety of secretions
The secreted substances may:
1) Remain deposited inside the secretory cell/tissue: for example in the case of the laticifers
2) It may be released from the secretory cell/tissue: for example, the resin ducts, nectaries, and secretory hairs
The distinction between ‘secretion’ and ‘excretion’ in plants is not always clear
Some of the secreted substances will take part in the metabolism of the plant
Even when not directly utilized by the plant in a physiological process, the secreted substances (secondary metabolites) play a role in the defence against viruses, bacteria, fungi, herbivores (e.g., resins, rubber, tannins), or as an attractant for pollinators (e.g., nectar)
Laticifers
Laticifers secrete latex which is released only if the plant is cut or damaged
Laticifers usually form a 3D network of tube-like cells within the entire plant
Evolution of laticifers is likely an anti-herbivory strategy because the latex is usually toxic
Latex also displays toxicity against insects, viruses, and fungi
Laticifers evolved independently in at least 60 plant families and latex is extremely diverse from a chemical point of view
Examples of plants include:
Papaveraceae (e.g., opium poppy); Euphorbiaceae (euphorbia, spurges), Apocynaceae (e.g., Asclepiaas, milkweeds, etc.) Asteraceae (e.g., Tarxcum - dandelions)
Articulated laticifers are composed of a series of cells joined together
Non-articulated laticifers, consisting on one long cell
Resin and Gum Ducts
Resin ducts are canals lined with secretory cells that release resins into the canal
Ducts form a complex 3D network through the entire body of the conifer plant:
Roots
Stems
Leaves
Flowers
Gum ducts are similar to resin ducts and may contain resins, oils, and gums
Usually, the term gum duct is more commonly used with reference to eudicots
Resins and gums have usually strong antibacterial properties protecting the plant in case of mechanical injury
Resins and Gums Have Been Used for Melinia
Used in many cultures as incense or medicinal
Nectaries (Nectar 'Glands’)
Nectaries produce a sugar-based secretion called nectar
They are typically associated with the flowers and their main purpose is to reward pollinators
In addition to different sugars, nectar contains amino-acids, volatiles and ions, alkaloids, polyphenols, etc.
From a practical point, nectar is the sugar source of honey
Extrafloral Nectars
When nectaries form on other organs than flowers, they are called extrafloral nectaries
In this case nectar is not associated with sexual reproduction, but it provides a nutrient source for animal mutualists (e.g., ants), which in turn ensure protection against herbivores
Secretory Trichomes (Glandular Hairs)
Secretory trichomes (hairs) = the product of secretion accumulates in a terminal cell or group of cells
Amazing morphological and chemical diversity
They have evolved many times in many different groups of plants and they serve many roles
Secretory hairs in Urticaceae (e.g., stinging nettle) are stinging
They have a distinct bulbous base and a stiff, translucent apex
The compounds producing the stinging sensation have been reported to be complex mixture of formic acid, histamine, acetylcholine, and 5-hydroxytryptamine
The tip of the stinging hair breaks off upon slight contact, leaving a sharp point that readily pierces skin and allows fluid contents of the hair to enter flesh through the body of the hair, which acts as a miniature hypodermic needle
Stinging nettles are not only edible when young, they are also delicious
Stems contain fibers that can be used as textile fibers
Stinging nettle is high in vitamin C, iron and potassium plus is used in herbal medicine as a treatment for many things, including allergies, formation of hemoglobin in red blood cells, as well as a diuretic
Also it makes an excellent replacement of spinach but you have to collect them when very young and tender (e.g., when plants have no more than 4-6 leaves)
The Cannabis “Resin” is Produced by Secretory Hairs Too
In Cannabis, glandular hairs secrete cannabinoids, terpenoids, etc.
Secretory Trichomes are Common in Many Plants
In Lamiaceae (mint family), they secrete volatile oils responsible for the flavor of culinary herbs such as mint, thyme, rosemary, sage, oregano, etc.
An epidermis with hairs like this can belong to the root
True
False