Pith is the spongy material found inside plant stems. When you are talking about citrus fruit, pith is the bitter white layer surrounding the sweet flesh of the fruit.
A pod is a long, thin seed casing found on plants like peas and beans. Pods are made from two halves joined together, and when the seeds inside are ripe they split open. The pods of many kinds of peas and bean can be eaten along with the seeds inside if they are picked early enough.
Pollen is made up of fine grains of male genetic material. Bundles of pollen are held inside flowers on the end of stamens, which you can clearly see in the picture. As insects or birds come to the flowers to eat nectar, they brush against the pollen and carry it to the stigma of another flower.
The rind of a fruit is the hard outer layer that you can't eat. Fruits like watermelon and oranges have rinds.
The roots of a plant are usually under the ground, holding the plant in place. The plant absorbs some food through its roots. The picture is of a tree where some of the dirt has washed away, so you can see its roots.
Sap is the thick, sticky fluid that is inside tree branches and stems. You can't see it until the plant is damaged in some way, like having a branch cut off or some kind of insect bores a hole into it. Many kinds of sap are sweet and are eaten by birds and insects. People collect the sap from maple trees to make maple syrup. Latex is made from sap as well.
Baby plants grow from seeds. If you bury a seed in soil and keep it moist, after a few days it will crack open and grow. Roots grow first from the seed, then the leaves. The seeds in the picture are sunflower seeds, just beginning to sprout.
Spores are the way some bacteria, fungi, ferns and algae reproduce. One spore can grow into a whole new organism. Spores are similar to seeds but they only need one parent not a male and a female parent like is needed to make seeds. Spores are also much smaller than seeds and do not have food reserves inside them.