The Nucleus

Abstract

The nucleus (Fig. 7-1) is a universal and prominent feature of all cells except bacteria and blue-green algae. A few cells, such as the sieve-tube elements of plants and the red blood cells of animals, may lose their nuclei when they become mature, but all cells have nuclei at some time in their lives. Situated in the cytoplasm, it may occupy a major proportion of the cell volume in young, rapidly dividing cells (Fig. 6-1). In older cells it comprises a smaller part of the total volume of the cell and may become pressed between the vacuole and the wall (Fig. 7-2). Although most cells of the vascular plant have but a single nucleus, some cells have two or more. The latex tubes are one example of a multinucleate cell in a vascular plant. In many nonvascular plants, the cells have more than one nucleus. There are numerous cases in which there are large numbers of free nuclei in a mass of cytoplasm, as in the slime molds, or where the number of nuclei are more limited, as in many algae.