Photo: Jill Clardy (flickr)
These are plants that have been meticulously trained to grow this way.
Growers start with a plant about a foot tall with a defined central stem. The plant is placed in a 10 inch pot, and a 3 to 6 feet stake is sunk in the soil until it hits the bottom of the pot, and it is then repeatedly tied tightly with soft florist tape to the stem of the plant to anchor and guide it so it grows perfectly straight.
All side shoots and branches are removed but the leaves at the top of the plant are pinched so that the top bushes out. Those top branches are also pinched until the top of the plant becomes very round and bushy.
The plant is re-potted as it grows taller and the pinching at the top continues to encourage and maintain the desired top-knot shape. A desirable top-knot takes about 3 years to form on a straight bare trunk.
A plant trained in this way is referred to as a standard.
Evergreen standards are seen as part of Holiday decorations in formal settings, and pairs are frequently used on either side of gates and doorways.
Roses and tropical plants such as Hibiscus and Lantana are sometimes trained in standard form to be used as accents.
Look for them in garden centers, catalogs and florists. They are expensive because of the time they take to evolve, but they are certainly stunning.