Nearly all conifers are evergreens, they retain their leaves throughout the year. Their needle-shaped leaves are adapted to any conditions. Photosynthesis can even occur on a mildly sunny day in winter. There are 575 species of conifers found in the northern hemisphere alone, making them the most familiar of all the gymnosperms. Sequoias and redwoods are among the oldest, largest and tallest organisms on earth.
Conifers exibit the most derived stele design, a eustele.
In the eustele, individual bundles of xylem and phloem are arranged to form a cylinder around a central pith. Between the xylem and phloem is the vascular cambium, a secondary meristem only about one cell layer thick. External to the vascular bundles is the cortex and finally the epidermis. A second menstematic layer called the cork cambium is located under the epidermis that gives rise to the periderm in woody plants. The roots of a conifer are also arranged in an evstele.