logo

Animation: Primary and Secondary Growth

by Pearson
2 views
Was this helpful ?
0
Primary and Secondary Growth A tree reaches its imposing size by combining primary and secondary growth. Let's review the two kinds of growth. Here you see the tip of a tree with the taper of the stem exaggerated for clarity. First, let's watch the tree grow, and then analyze the growth regions. Notice that some parts of a tree undergo primary growth at the same time that other parts undergo secondary growth. If the roots were visible, you would see the same thing in the roots. Primary growth occurs in young leaves and elongating parts of the stems and roots. Apical meristems, or regions of rapid cell division, provide cells for primary growth. Secondary growth occurs in regions of stems or roots that have completed their elongation. Leaves rarely have secondary growth. Secondary growth doesn't occur in many short-lived eudicots and most monocots, such as palms and grasses. Secondary growth occurs when a lateral meristem called the vascular cambium adds xylem and phloem to the stem. This picture shows a segment of a tree trunk with the inner tissue zones raised for visibility. The vascular cambium forms secondary phloem toward the stem's surface and secondary xylem that is wood toward the stem's center. That's the main reason why tree trunks thicken with age. Secondary growth also adds cork that protects and waterproofs stems and roots. Cork is made by one or more lateral meristems called cork cambiums, which occur outside the secondary phloem. Cork cambiums work at the same time as the vascular cambium, though we don't show that here. Besides making cork, cork cambiums often make extra storage tissue toward the inside of the stem, though we don't show that here either. The vascular cambium makes xylem and phloem by dividing its cells repeatedly, with outward cells becoming phloem and inward cells becoming xylem. Why doesn't this dual operation consume the cambium? To maintain itself, the vascular cambium alternates between making xylem and phloem. After each cell division, one cell stays meristematic, while the other differentiates, acquiring the features of a xylem or phloem cell. More divisions produce xylem than phloem. That's part of the reason why wood, or the secondary xylem, becomes much thicker than bark, which contains secondary phloem and cork.
Primary and Secondary Growth A tree reaches its imposing size by combining primary and secondary growth. Let's review the two kinds of growth. Here you see the tip of a tree with the taper of the stem exaggerated for clarity. First, let's watch the tree grow, and then analyze the growth regions. Notice that some parts of a tree undergo primary growth at the same time that other parts undergo secondary growth. If the roots were visible, you would see the same thing in the roots. Primary growth occurs in young leaves and elongating parts of the stems and roots. Apical meristems, or regions of rapid cell division, provide cells for primary growth. Secondary growth occurs in regions of stems or roots that have completed their elongation. Leaves rarely have secondary growth. Secondary growth doesn't occur in many short-lived eudicots and most monocots, such as palms and grasses. Secondary growth occurs when a lateral meristem called the vascular cambium adds xylem and phloem to the stem. This picture shows a segment of a tree trunk with the inner tissue zones raised for visibility. The vascular cambium forms secondary phloem toward the stem's surface and secondary xylem that is wood toward the stem's center. That's the main reason why tree trunks thicken with age. Secondary growth also adds cork that protects and waterproofs stems and roots. Cork is made by one or more lateral meristems called cork cambiums, which occur outside the secondary phloem. Cork cambiums work at the same time as the vascular cambium, though we don't show that here. Besides making cork, cork cambiums often make extra storage tissue toward the inside of the stem, though we don't show that here either. The vascular cambium makes xylem and phloem by dividing its cells repeatedly, with outward cells becoming phloem and inward cells becoming xylem. Why doesn't this dual operation consume the cambium? To maintain itself, the vascular cambium alternates between making xylem and phloem. After each cell division, one cell stays meristematic, while the other differentiates, acquiring the features of a xylem or phloem cell. More divisions produce xylem than phloem. That's part of the reason why wood, or the secondary xylem, becomes much thicker than bark, which contains secondary phloem and cork.