5 Steps to a 5: AP Biology 2020 by Mark Anestis

Author:Mark Anestis
Language: eng
Format: epub
Publisher: McGraw-Hill Education
Published: 2020-01-04T16:00:00+00:00

Structure of Stems

Again, let’s travel from the outside in and discuss the basic structure. The epidermis for the stem provides protection and is covered by cutin, a waxy protective coat. The cortex of a stem contains the parenchyma, collenchyma, and schlerenchyma cells mentioned earlier in this chapter. You’ll notice that there is no endodermis in the stem because this portion of the plant is not involved in the absorption of water. As a result, the next structure we see as we move inward is the vascular cylinder and our friends the xylem and phloem.

A term to know is the vascular cambium, which extends along the entire length of the plant and gives rise to secondary xylem and phloem. Over time, the stem of a plant will increase in width because of the secondary xylem produced each year.

Another term to know is the cork cambium, which produces a thick cover for stems and roots. This covering replaces the epidermis when it dries up and falls off the stem during secondary growth, forming a protective barrier against infection and physical damage.

The growth of plants is not a continuous process in seasonal environments. There are periods of dormancy in between phases of growth. Have you ever seen the rings of a tree after it has been cut down? These rings produced each year are a window into the past, and give insight into the amount of rain a tree has encountered in a given year. The wider the ring, the more water it saw.

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