s they are the same color as the petals (in which case the term tepal is applied since sepals and petals look so much alike).

separation Splitting of the cytoplasm by cytokinesis (= cytokinesis).

severe combined immunodeficiency (SCID)  A genetic disorder in which afþicted individuals have no functional immune system and are prone to infections. Both the cell-mediated immune response and the antibody-mediated response are absent.

sex chromosomes  The chromosomes that determine the sex of an organism. In humans, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Chromosome that determines the gender (sex) of the individual. Human males have a large X and a smaller Y sex chromosomes, while human females have two X sex chromosomes. PICTURE

sex hormones  A group of steroid hormones produced by the adrenal cortex. Hormones that are produced in the gonads and promoted development and maintainence of the secondary sex characteristics and structures, prepare the female for pregnancy, and aid in development of gametes. Males produce testosterone, while females produce estrogen and progesterone.

sex linkage  The condition in which the inheritance of a sex chromosome is coupled with that of a given gene; e.g., red-green color blindness and hemophilia in humans. Traits located on the X-chromosome.

sexual reproduction  A system of reproduction in which two haploid sex cells (gametes) fuse to produce a diploid zygote. PICTURE

shoot  The plant stem; provides support for the leaves and þowers; one of the three major plant organs; also referred to as the shoot system. PICTURE

short-day plants  Plants that þower during early spring or fall when nights are relatively long and days are short; e.g., poinsettia and dandelions.

sickle cell anemia Human autosomal recessive disease that causes production of abnormal red blood cells that collapse (or sickle) and cause circulatory problems.

sieve cells Conducting cells in the phloem of vascular plants. See sieve elements

sieve elements  Tubular, thin-walled cells that form a system of tubes extending from the roots to the leaves in the phloem of plants; lose their nuclei and organelles at maturity, but retain a functional plasma membrane. PICTURE 1 PICTURE 2

sieve plates  Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of sieve tub