Plants and animals owe their regenerative and replaceable properties to a class of cells known as stem cells. Plants specifically have 4 types of stem cells, also called meristems, scientists have identified and studied. First, is the shoot apical meristem (SAM). SAM's can be found at the tips of growing shoots as small translucent clumps. They're encased by a protective cover of the differentiated cells they've recently produced as to prevent any damage. Second, are auxiliary meristems. These meristems determine the sites of leaf growth and leaf categorization. If off the main shoot there is a leaf and an auxiliary meristem between the two, this leaf can be classified as a simple leaf. A shoot next to an auxiliary meristem next to a branch with leaves classifies the branched leaves as compound leaves, where each individual leaf is a leaflet. A shoot next to an auxiliary meristem next to a branch off of which smaller branches appear with leaves is classified as a doubly compounded leaf. Third, is root apical meristems (RAM). RAM's appear at root tips with a hard shell protecting the absolute tip. This is because, unlike the SAM, the roots are more susceptible to damage. Similar to the SAM's these cells grow the roots out while also producing more meristematic cells. Fourth is cambium cells. There are two cell types that fall under the cambium denomination; the cork cambium, and the vascular cambium. The cork cambium includes the bark and phloem on a plant near the outside. The vascular cambium includes some of the phloem and xylem cells on the inside of the plant.