PLANTS - COLOR AND TEXTURE
Vegetables and fruits can be either crispy, soft, or dry, depending on how much water they contain and how their cell walls look like. The reason is that plant cells contain large baloon-like vacuoles that are filled with water and therefore stretch out the cells that will press against the cell walls that run between the cells. This gives the plant this fresh-looking tensed look, as opposed to when a plant has limited access to water and wilt. A good property that plants have, if they become too dehydrated, is that you can soak them in water and they will regain their tension and moisture within a few hours (unless they are really dead). A plant that is moist is desirable in cooking, since this gives it the crispiness and juiciness that people look for when enjoying the plant food. Some plants, especially fruits, are not crisp but rather mealy or dry, and this is a result of a slightly different composition of the cell walls than these in crisp vegetables. If the cell walls are weak, our chewing will rather break them apart and not cause as much bursting of vacuoles (which creates the crisp feel) - instead, the vacuoles will break more gently, and create a feel of melting in the mouth along with the cell walls' mealiness.
Cell walls then play a central role in how texture of a plant will be. Cell walls are also changed in cooking, which will of course change the plant's texture. Cooking will dissolve more or less of the cell wall components, since they are in part water-soluble. This will make the plant more soft, and the longer we cook it the more soft it will be. This is why chefs often try to refresh a cooked vegetable in ice water, which basically works like this: when the plant is cooked sufficiently well, but we still want to retain some of the crispiness that is left in it, we need to stop the cooking process abruptly. That's why we put the plant in cold water for a moment.
Besides texture, plants are well-known for their colors. Green, yellow, red, orange, are the common nice colors of vegetables and plants that can be certainly put to use in a dish just for esthetical reasons. Colors of the plants come from certain molecules that are able to absorb sunlight at specific wavelengths (colors) and thus leave out and reflect to our eyes the part that is not absorbed. The green color comes from chlorophylls which are contained within the cells, but once they are cooked, their color may start to change depending on how high the temperature is and how long you cook. The yellow and red colors come from molecules called retinoids, and are found in veggies like carrots or peppers. One of the retinoids, beta-carotene, is essential for us in order for our bodies to create vitamin A. Carotenoids function as protectors against damaging light wavelengths, and are usually high in plants that are also very green otherwise. Even other molecules, betains and antoxanthins, provide color in certain plants.
Some vegetables and fruits, like artichokes or potatoes, become quickly browned when exposed to air or if just damaged. This is because some oxidative enzymes become exposed and active, creating the discoloration. Several old techniques are known to prevent this, one is to smear the veggie with lemon juice, or just by putting the veggies into cold water. These procedures will inhibit the activity of the oxidative enzymes.