분열조직과 식물 성장

Unlike most animals, plants can grow throughout their lives. Plant growth is facilitated by meristems, tissues containing undifferentiated cells that can perpetually divide—akin to animal stem cells.  

All vascular plants undergo primary growth, which lengthens roots and shoots. 

Primary growth begins in apical meristems, located at the tips of stems and roots. Here, undifferentiated cells divide. Some daughter cells remain in the meristem and go on to divide again. Others partially differentiate into primary meristems.

The primary meristems—the protoderm, ground meristem, and procambium—differentiate into distinct primary tissues.

The protoderm becomes dermal tissue, the protective outer layers. The ground meristem develops into ground tissue, which makes up most of the body of a young plant. The procambium forms vascular tissue, including xylem and phloem.

In addition to primary growth, woody plants also undergo secondary growth. Secondary growth widens roots and stems, but rarely leaves.

Two lateral meristems contribute to secondary growth: the cork cambium and the vascular cambium.

The cork cambium produces cork to the outside and phelloderm to the inside, forming the periderm. 

The vascular cambium is located between the primary xylem and primary phloem. Here, secondary growth generates secondary phloem to the outside and secondary xylem, also known as wood, to the inside.

In many trees, this activity produces annual growth rings, which scientists can use to study climate.

Wood grown in the spring contains cells with larger diameters and thinner walls than wood produced later in the growing season. A year of growth, therefore, manifests as a distinct ring.

Since trees grow well in warm environments with plenty of water, thick rings indicate warm, wet years and thin rings indicate cold or dry years.

Overall, primary growth allows plants to extend their shoots in search of light and their roots to gather moisture and nutrients. Meanwhile, secondary growth provides structural support to accommodate the lengthening of woody plants.