lateral meristem

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A meristem in vascular plants, such as the cambium, in which secondary growth occurs.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany Meristem forming a cylinder within some plants' organs (primarily stems, branches and roots) and allowing them to increase in girth (lateral growth).