|Plant Taxonomy (BIOL308) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; firstname.lastname@example.org; http://www.employees.csbsju.edu/ssaupe/|
I. General Comments
Propagule production, protection, and dispersal. The fruit OR the seed can be the propagule (the dispersal unit is sometimes called the diaspore) and can be disseminated by various vectors including wind, water, and animals.
Fruits = ripened, mature ovary and contents (seeds). Frequently associated with accessory structures (such as receptacle tissue). Seed = ripened ovule.
C. Fruits vs. veggies
Botanically, fruits are derived from the ovary or reproductive parts of the plants. Vegetables are derived from "vegetative" parts, such as leaves, roots and stems. Simple, right? So, is a zucchini a fruit or vegetable? How about cauliflower? or tomato? or squash? Many foods that we consider vegetables are actually fruits, and vice versa. The problem arises because "fruit" is commonly used to describe a food that is usually sweet and eaten as dessert or perhaps salad, whereas vegetable is used to describe a food, usually green, that is eaten during the main course of a meal.
D. Seeds vs. Fruits
These terms are often incorrectly applied (from a strict botanical sense) by the non-botanist. For example, sunflower seeds and corn grains are actually fruits. Conversely, seeds like coconut, are often considered to be a fruit. Take-home-lesson: fruits have seeds, vegetables do not...the seeds are inside the fruit.
II. Life Cycle Review
We will go over the standard angiosperm life cycle. Included in our discussion will be: alternation of generations, sporophyte, gametophyte, ovule, double fertilization, endosperm, seeds. For a review of the angiosperm life cycle, check out my notes from Biology 116. Points to note include:
- Plants have sex (meiosis & fertilization)
- Meiosis occurs in the stamen (anther) and pistil (ovule)
- The female gametophyte is in the ovule, protected from the environment
- Pollen is the male gametophyte
- Pollination is the transfer of pollen from anther to stigma
- Pollen germinates on a compatible stigma
- Two sperms develop in the germinating pollen grain
- Fertilization occurs in the carpel (ovule)
- Fertilization and pollination are different events
- Plants have two fertilizations - one to produce the zygote (embryo) and the other to give rise to food storage tissue (endosperm)
- Plant embryos are not miniaturized adults (as in animals)
- Plant embryo development is discontinuous (to allow time for dispersal)
- Plants exhibit an alternation of generations having a distinct haploid (gametophyte) and diploid (sporophyte) stage
- The gametophyte phase of flowering plants is short-lived; the sporophyte is the dominant phase
- After fertilization, the ovule develops into a seed
- The ovary develops into a fruit
- For every seed there was one ovule in the ovary
- Each seed required one pollen grain for development
- Each seed requires 2 sperm (present inside the single pollen grain) for development
Thus, to produce a watermelon that contains 1000 seeds, it requires one flower, with one ovary, containing a minimum of 1000 ovules, each of which was fertilized by a separate pollen grain (1000 total) containing 2 sperm (2000 sperm total).
The wall of the fruit is called the pericarp. Derived from the ovary wall. Comprised of three layers that may or may not be distinct: exocarp (outer layer of pericarp), mesocarp (middle), endocarp (inner). The seed coat is derived from the integuments (outer layers) of the ovule.
The fruit may include non-ovarian tissue from structures such as the receptacle, calyx, corolla, and even bracts.
Classification of fruits can be done in various ways, none completely satisfactory. Fruits can be classified on the basis of the:
IV. Fruit Types - or, Fun at the Grocery Store
In class/lab we will show and discuss examples of a variety of fruits. For fun, consider a trip to the store to check out some fruits. Click here for details about each fruit type. Also, Gink & Go at Lunch has some information on fruit types/structures and the PowerPoint presentation from class is posted in the Public Folders.
V. Seed structure
A seed is essentially a sleeping baby in a suitcase with his/her lunch. The three major parts of a seed are:
A. Seed coat (also called the testa) - the "suitcase"
It is derived from the integuments of the ovule. Thus, it is maternal tissue (the only part of the seed that is not part of the "new" generation). The seed coat is primarily for protection. The seed coat may have hairs (i.e., cotton) or other appendages. The hilum is the point where the seed coat detached from the ovary (like our bellybutton). Near the hilum a micropyle can often been seen (point of entry of the pollen tube into the ovule). The stalk that connects the seed to the fruit is the funiculus (like our umbilical cord). The hilum or funiculus may be enlarged into a fleshy aril.
B. Embryo - "the baby"
Derived from the zygote. The embryo has a region that will give rise to roots (radicle); a region that will produce the shoots (epicotyl/plumule), a stem axis (hypocotyl) that connects the root end to the shoot end; and embryonic leaves called the cotyledons or seed leaves. The appearance of these structures vary with the species. There are two basic patterns; monocot embryos and dicot embyros. As the names suggest, perhaps the major difference is that monocots have a single cotyledon vs. two in dicots.
C. Food Source - "the lunch".
The germinating seed is essentially heterotrophic - it subsists off of its own stored food reserves until it becomes photosynthetically competent. Thus, it must contain a nutrient source. This nutrient source can take two forms: (a) endosperm (derived from the fusion of one of the sperm with the fusion nuclei). Endosperm surrounds the embryo and is common in grasses; or (b) in other species, the endosperm is reabsorbed during development and stored as starch, protein or oil in the cotyledons (like beans).
The main role of a fruit is dispersal of the offspring (seeds) to a new environment. Since plants are non-motile, the evolution of dispersal mechanisms for the offspring was of prime importance. Sometimes, the fruit releases the seeds (dehiscent) which are the actual dispersal agent. For example, the milkweed pod (actually a follicle) splits open to release the plumed seeds. Botanists call the actual dispersal unit, whether seed or fruit, a diaspore. There are many means by which diaspores are dispersed:
A. Wind. Fruits/seeds can have a variety of modification for wind dispersal including:
Parachutes - such as the plumed or wooly seeds or fruits of milkweed, cottonwood, cattails, willow, goldenrod, dandelion, goat's-beard.
Wings - in fruits/seeds like maple, catalpa, ash, birch, tulilp-tree, elm, trumpet-creeper, pines, spruces, arborvitae. These fruits/seeds often spiral to the ground like a helicopter.
Sails - that catch the wind like basswood, dock, and birch
Tumbleweeds or rollers - in these plants, the entire shoot breaks off at the crown and rolls (i.e., Salsola kali)
Dust - these seeds are so tiny that they are easily blown about (i.e., orchid seeds)
Shakers - in these plants the wind batters the plant throwing the seeds such as in poppy
B. Water. This mode includes such agents as rivers, floods, ice, sea (coconut, mangrove) and rainwash dispersal such as mitrewort. Seeds of the latter species are essentially splashed out of a cup-like fruit by a raindrop. Wild rice seeds are dropped into the water and dispersed.
C. Animal Dispersal. There are several ways animals disperse propagules:
D. Mechanical Dispersal. Explosive dehiscence in plants such as Impatiens (touch-me-nots), wild cucumber, witch hazel and the throwers like geranium.
E. Humidity Sensitivity. Some fruits/plants are sensitive to humidity which aids in their dissemination. For example, the fruiting umbels of Queen Anne's lace opens and closes with the humidity and the awns of needle-grass twist to help plant the seed.
F. Humans. Diaspores have been mostly unintentionally dispersed by: (a) accident
(via vehicles, persons, trains); (b) among crop seeds.
10/02/2007 / � Copyright by SG