Roots are annual, biennial, or perennial, depending on their length of life. In the same way the stem-growth is divided into two main classes or divisions, the herbaceous stems living only one year, and the woody plants living more than one year, and in some cases one hundred years or more. When of small growth woody plants are classed as shrubs. But it often happens that shrubs in one climate are trees in another, and herbaceous plants in one climate become woody-stemmed trees in another. As an instance, we have seen the castor-oil bean grown into a woody-stemmed tree in Cuba thirty or more feet in height and with a stem ten inches in diameter. As to mode of cell-growth, our cultivated plants are divided into two main classes or divisions, the "exogens" and the "endogens," meaning "outside growers" and "inside growers." Corn, asparagus, palms, ferns, and many tropical trees are inside growers or endogens. In this division the new cell-growth is mingled with the older tissue, and growth of the stem is by distension or pressing outward from the inside. This class of stems does not show the bark, wood, and pith of the outside growers. The exogenous division includes all our northern trees.