Classification and technicalities (get this out of the way)
Lets start off by talking about plant classifications based on structure.
Some plants have no vascular system at all. There is very little capillary action taking place in these plants and the plant parts all remain very low to the root system. Many of these plants reproduce by spores and root branchings. This class of plants falls under "non-vascular" plants and includes mosses and liverworts. These fall outside the scope of my own knowledge and are excluded from this particular discussion for now.
Next in the evolutionary chain of plants are the fern-type plants. It is theorized that somewhere along the line, non-vascular plants and fungi formed a symbiotic relationship whereby fungi helped move water and nutrients up into the "leafy" parts of the plants. This relationship allowed plants to grow upward to reach more sun while still being able to retrieve nutrients from the soil. Plants gained the ability to overgrow smaller plants and gain dominance over their mossy counterparts. They did however retain their reproductive system of spores and root branchings. These plants have vascular systems and fall under the second class. If your plant is clearly not moss, and has veins and leaf systems, it falls into this category. This class is very broad and includes most plants as we know them.
Non-vascular: Having no capillary parts to move water and nutrients up from the root system into the leaf system and vice-versa. This is the most basic classification of plantae.
Vascular: Having a veinous network of capillary tubes and specialized cells for moving water and nutrients up from the root system, into the plant structure above ground or sugars that have been processed through photosynthesis back down into the roots.
All plants in the world will fall into one of these two categories. The vascular plants class is further divided into more categories based on how they reproduce. Fern type plants do not produce flowers. Above the ferns are the flowering plants. Where ferns produce fronds with glands and spores, other plants produce insect-attracting flowers and have male and female parts. Flower parts will be covered in a moment. Outside of ferns, there are two types of vascular plants: those with parallel venation, and those with "webbed" veination.
Hold a leaf up to a light and look "through" it. If none of the veins in the leaves touch each other, your plant falls into the Liliopsida (Lily) Class. If the veins branch multiple times and often have veins connecting veins, your plant falls into the Magnoliopsida Class. Sprouts from either of these classes are called "Cotyledons" meaning "seed leaf". How the plant emerges from it's seed is an important detail to notice.
Ever looked at a pea sprout in the garden? A taproot emerges from the seed-pea and pushes up on the seed. Each half of the seed becomes a leaf. These are called "dicot" plants and all of these will fall under the Magniolopsida class. Other seeds remain in the ground and send up leaves from the seed, while it remains in the ground. All of these plants are called "monocot" and all of them are in the Liliopsida class.
Cotyledon: First emerging leaves from a seed. think "sprouts".
Monocot: A plant in the Liliopsida family having grasslike leaves with parallel veination.
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Dicot: A plant in the Magniolopsida family having broad, often "shaped" leaves with webbed veination
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Each of these classifications are further divided into Orders. The order includes many plants with very similar characteristics, but is not very specific. There are no useful patters of the plant orders for identifying a particular plant. For example the Rose Order includes dewberries, and plums, and pears. While at first glance, the entire plants are extremely different, the flowers all share very similar characteristics.
Orders are further divided into Families. This is where details that will be covered in the next discussion will help you most. All plants in a particular family will have very similar characteristics which can be used to identify a plant. Any random plant is best "lumped" into a family for identification. By looking at the details of the flower and plant structure you can "classify" your plant by family, so that you know what plants NOT to look at for ID. This skill alone will speed up the identification process, by immediate elimination of invalid options.
Families are further divided into classifications based on details. Below a Family, is the Genus, Which is like a surname, and below that is the Species, which is like a first name (for people). A genus is a smaller "lump" of plants with more similar characteristics, and species is quite specific to a particular plant, although some species have "variations" that do not change the plant enough for it to fall into a new plant species. An example is flower color, which is quite arbitrary if you have ever looked through a Peterson Guide. Many of the plants included in the field guide are repeated in different "flower color" sections, and "variations" are the reason.
While we only covered a few definitions here, it is a lot of information to take in and remember. We'll stop here for now so you can absorb it and go out and look at plants and notice these few details.
Our next stop is actual structure of the plant itself. Root, stem, leaf, flower, stalk, etc. all have their place in plant identification and it only gets more complicated from here. Study what we've covered, go out and look at examples, and take notes on what you observe.
Plant Parts: Fruits and seeds
Seeds and fruits come in all shapes and sizes. First we need to understand that pollen from the anthers of the stamens, comes into contact with the style of the pistil and travels down the pistil tube into the ovary to fertilize seed-eggs (ovules). This ovary is the part that matures after the petals fall off the flower, and it forms a "fruit". Fruit is a broad term that represents all seeds. There are simple (like peaches) and complex fruits (like strawberries), just as there are with leaves and other parts of plants. Lets back up for a second, back to the ovary and pistil.
Ovaries sometimes have a single chamber, and more often (through plant adaptation) have multiple chambers called carpels. What we see in many of the plants we encounter is (backing up again) radial symmetry. Often if a plant has 4 petals, and 4 sepals, and a single pistil with 4 styles, guess how many carpels it has... That's right, 4! Some plants have multiple pistils and ovaries, all of which are simple, and occur in bunches. Roses and dewberries have many simple pistils and ovaries and each seed occurs in a single carpel (unicarpellate). A pea or bean pod is a single carpel with many seeds. Many plants have simplified their reproductive parts into compound forms. A dewberry or blackberry is made of many smaller berries, each containing one seed. They are all in a dense cluster that looks like one large berry, but they are not!
The simplest fruits are achenes. An achene contains only one seed. The seed itself is surrounded by a (or several layers that comprise the) pericarp, the "meat" of the fruit. A strawberry is a great example of a fruit covered with achenes. Each seed on the oustide of a strawberry is connected at a single point along the outside of the fruit and none of them have meat inside. The part you eat is really not even part of the seeds. It is more of a passive, starch-storage device. Such a fruit is called aggregate. It has many mature ovaries attached to the recptacle of a single flower. Blackberries are another good example of aggregate fruits. Dandelion seeds are excellent examples of achenes.
Blackberries are also good examples of a drupe. Peaches and cherries all have a stony pit, juicy or pulpy "meat" and a "skin". These 3 layers of the pericarp make a drupe distinct from an achene, which has no meat between the seed and skin. Cherries are excellent examples of a drupe. We think of them as "berries" but that is a technical misinterpretation, because we don't think of a peach as a berry. A berry actually has two or more seeds inside, like a muscadine! The other characteristics still apply, but note that a muscadine has many seeds inside and is actually a berry, while a cherry is a drupe! Sumac berries are persistent, meaning they remain attached after the rest of the foliage has fallen away.
A nut has no outer pericarp and the "meat" is inside the shell. A nut has only one seed inside, and is not juicy or mealy or pulpy. Often, nuts are valuable for their oil content. Often these oils are in the forms of amino acids and fats that are very good sources of protein and energy. Most nuts do not open on their own and are called indehiscent. If a seed resembles a nut, but several comprise the whole, as with mints and mallows, they are called nutlets. Nutlets are often borne together and look like one whole entity, but they clearly seperate from each other into smaller parts.
Backing up for a second, to peas and beans, you notice they are unicarpellate (having only one chamber containing many seeds). At maturity the seeds are dry and the carpel opens along a particular line on it's own. The elongated ovary wall containing the seeds is called a pod. Because it opens to unleash it's seeds, all of it's own accord, we call it dehiscent.
When you eat an apple, you notice the outer skin, mealy pulp, and an inner "papery wall" that seperates it all from the seeds. An apple is a pome, and I have to apologize for lack of a better definition. Pears are another good example of pomes. (Any better definition gets into some really technical terminology.)
Then there are the more complex (many parts) fruits, such as pine cones. Surely you have noticed the male pollinators that a pine tree bears. Pine trees have unisexual flowers, meaning the male and female reproductive parts occur seperately. The male parts, and the female cones are called catkins. Many other plants reproduce by catkins such as this.
Maple trees reproduce by "winged" seeds called keys. These keys are called samaras. Pine tree cones, upon reaching maturity, release their solitary keys, which "helicopter" down to the ground.
I hope this will not add to any confusion regarding the descriptions of fruits. It has taken me quite a bit of study and repetition to understand the differences in all these definitions, and I hope my "plain Jane" way of explaining each one will make sense to any newcomer to the world of botanical descriptions. I have tried to use very common examples so there should be no real need for pictures here.