694 - Appendix 2 



a receptor that is not a neuron; its axon usually 

 runs into the central nervous system. 



Si pals. The separate parts of the calyx. 



Sex chromosomes. A particular pair of chromosomes 

 that (normally) determines the sex of an organ- 

 ism. 



Sex-linked gene. Genes in the sex chromosomes, 

 which are therefore linked in heredity to the genes 

 determining sex. 



Sexual reproduction. Reproduction involving the 

 fusion of two cells (gametes) , especially of their 

 nuclei. 



Sieve tubes. Tubular structures in the phloem por- 

 tion of vascular bundles, composed of elongate 

 tells whose walls are perforated by fine pores giving 

 direct protoplasmic continuity from cell to cell. 



Solute. A substance dissolved in another. 



Solution. A mixture in which one or more sub- 

 stances are dispersed in the form of separate mole- 

 cules or ions throughout another (usually liquid) 

 substance. 



Solvent. The substance in which another is dis- 

 solved. 



Soma. The somatic cells collectively. 



Somatic cells. The body cells of an organism, 

 which do not stand in the line of ascent of the re- 

 productive cells, (rf. Germ cells.) 



Species. A taxonomic group, supposed to be a group 

 of individuals that do not differ from each other 

 more than offspring of the same parent may do. and 

 all of which may interbreed freely; in practice, how- 

 ever, the lines between species are often drawn ar- 

 bitrarily. 



Spermarv. The male gonad, in which the sperm 

 cells are formed. 



Spermatids. Germ cells of the male after the meiotic 

 divisions but before assuming the specialized form 

 of the sperm cells. 



Spermatocytes. The auxocytes of the male, which 

 give rise to the spermatids. 



Spermatogonia. The gonia of the male, which give 

 rise to the spermatocytes. 



Spermatozoon. A sperm cell. 



Sperm cell. A small, usually motile gamete, contain- 

 ing very little cytoplasm and no yolk. (rf. Ec.c. 

 cell.) 



Spermiducts. The male gonoducts. 



Spherical symmetry. Symmetry built around a 

 point. In a spherically symmetrical body (e.g., a 

 ball), any plane that passes through the cen- 

 tral point will divide the body into symmetrical 

 halves. 



Spinal cord. 7ne central nervous system, exclusive 

 of the brain, of vertebrates. 



Spinal nerves. Nerves arising from the spinal cord. 



Spindle. A spindle-shaped group of fibers, converg- 

 ing toward opposite ends of the cell, yvhich appears 

 during mitosis and meiosis. 



Sporangium. A hollow structure in yvhich the spores 

 are formed. 



Spore. An asexual specialized reproductive cell. 



Sporocyte. A cell from which four spores are formed 

 by the two meiotic divisions. 



Sporophyll. A modified leaf bearing sporangia. 



Sporophyte. The asexual generation of a plant that 

 has an alternation of generations, (rf. Gameto- 

 phyte.) 



Stamen. A microsporophyll of a flowering" plant. 



Starch. A complex carbohydrate (polysaccharide) , 

 the chief stored food substance of most green plants. 



Stf.apsin. An enzyme secreted by the pancreas, which 

 hydrolyzes fats and some other lipids. 



Stereo i korisM. Movement whose direction is deter- 

 mined by contact with a solid body. 



Sterols. Complex lipids, not susceptible to hydrol- 

 ysis, found in all cells. 



Stigma. A moist, sticky surface at the tip of the pistil, 

 on which the pollen grains germinate. 



Stimulus. Any change of external or internal condi- 

 tions that results in a change in the activities of the 

 organism, (rf. Response.) 



Stoma. An opening from the intercellular space of 

 a leaf or stem to the outer air, surrounded by two 

 guard cells. 



Style. The elongate portion of a pistil, between the 

 stigma and the ovule case, through yvhich the pollen 

 tube grows. 



Substance. Am material composed of only one kind 

 of molecule, (rf. Mixture.) 



Substrate. A substance acted on by a catalyst. 



Sucrose. Common sugar, a disaccharide found in 

 many' plants; e.g.. sugar cane, sugar beets, etc. 



Sugar. Any monosaccharide or disaccharide. 



Suspension. A dispersion consisting of solid particles 

 in a liquid medium. 



Swarm spore. A motile spore, characteristic of some 

 thallophy tes. 



Symbiosis. A mutually beneficial relation between a 

 parasite and its host. 



Sympathetic system. That portion of the autonomic 

 nervous system which is connected with the anterior 

 and middle regions of the spinal cord, and whose 

 ganglia lie close to the spinal cord or in the ab- 

 dominal cavity. 



Synapse. The point of contact between two succes- 

 sive neurons. 



Synapsis. The intimate conjunction of homologous 

 chromosomes that occurs during the prophase of 

 the first meiotic division. 



Syncytium. A continuous mass of protoplasm with 

 several or many nuclei. 



Synthesis. The formation of a more complex sub- 

 stance from simpler ones. 



Taxonomic Pertaining to the classification of organ- 

 isms. 



Tflolfcithal ec.c. An egg cell with abundant volk 

 concentrated toward the vegetal end of the cell. 



Telophases. The final stages of mitosis, during 

 which the daughter nuclei are formed and the 

 cytoplasm is divided. 



Testis, A spermarv. 



Tetrad. A group of four chromosomes consisting of 

 a pair of homologous chromosomes each having 

 divided into tyvo daughter chromosomes; formed 

 after synapsis. 



Thalamus. The part of the brain just behind the 

 cerebrum; an important intermediate station be- 



