being 1 pure dominant to 2 hybrids to 1 pure 

 recessive. 



selection: in breeding, the choosing of certain or- 

 ganisms with desirable characters to be the 

 parents of future generations. 



semicircular canals: three arches lying in different 

 planes in the inner ear; concerned with the 

 sense of balance. 



sepals (see'pals) : leaflike parts forming the outer- 

 most circle of most flowers, usually green; to- 

 gether they form the calyx. 



serous (sear'us) membrane: very thin, smooth 

 membrane lining the blood vessels and body 

 cavities in man and other animals; also cov- 

 ers organs. 



serum: in general any watery animal fluid; in 

 clotting, the blood serum is the clear liquid 

 which separates from the clot ; when contain- 

 ing special immune substances serum is used 

 for inoculation. 



sex cells: male and female reproductive cells; 

 eggs and sperms; contained in male and fe- 

 male sex organs. Primary sex cells: the cells 

 which eventually in maturation develop into 

 sex cells. 



sexual reproduction: commonest method of repro- 

 duction among both simple and complex ani- 

 mals and plants in which the life of the new 

 individual starts with the union of two cells 

 (gametes) . 



sheeting: floating or steady downhill moving of a 

 well-soaked layer of topsoil. 



shrub: woody, perennial seed plant with stems 

 branching just above the ground; usually 

 smaller than trees. 



sieve (siv) tubes: tubes in the phloem consisting 

 of living cells through which manufactured 

 foods pass. Sieve plates, or cell walls with 

 holes, form at the top and bottom of each 

 cell of a sieve tube. 



slip or cutting: piece of a stem or leaf suitable 

 for propagation of a new plant. 



somatic (so-mat'ic) cells: cells in a higher ani- 

 mal that make up the body as distinguished 

 from the sex cells. 



somatoplasm (so-mat'oh-plasm) : protoplasm of 

 the somatic cells as distinguished from germ 

 plasm. 



spawning: releasing of eggs into the water by 

 aquatic animals. 



species (spee'shees) : in classification the subdivi- 

 sion of a genus; often the final division. 

 Every species has a name of at least two 

 words, the first of which is the genus 

 name. 



spermary (sperm'a-ree) : testis; male reproduc- 

 tive organ in animals. It produces sperm 

 cells which pass to the exterior through sperm 

 ducts. 



spermatophytes (sperm-at'oh-fites) : members of 

 a large group (phylum) of plants that pro- 

 duce seeds; "highest" plant group. 



spinal column: series of vertebrae in the region 

 of the back which enclose the spinal cord. 



spinal cord: nerve tissue which is the continuation 

 of the brain and which lies within the spinal 

 column. 



spiracles (spy're-kels) : in insects and some other 

 arthropods the openings on the surface of the 

 body leading into the breathing tubes or 

 tracheae. 



spirillum (spy-ril'lum) : spiral-shaped bacterium. 



spleen: an abdominal organ in higher animals; 

 stores blood, especially red cells, makes cer- 

 tain white blood cells, and destroys old red 

 blood cells. 



spongy parenchyma (per-en'kim-ma) : irregular 

 cells surrounding air spaces just above the 

 lower epidermis of a leaf. 



spontaneous generation, theory of: living things 

 can arise from dead matter; now disproved. 



sporangium (spore-an'jee-um) or spore case: 

 spore-forming organ in many plants. 



spore: in reproduction, a one or two-celled body 

 formed sexually or asexually in plants and 

 some protozoa; in some cases it has a re- 

 sistant wall. Also a bacterium in a stage in 

 which it has a thick, resistant wall. 



sporulation (spore'you-lay'shun) : multiple divi- 

 sion of a cell resulting in the production of 

 spores, as in the bread mold; form of asexual 

 reproduction. 



stamen (stay'men) : organ of a flower that pro- 

 duces the pollen; one of the essential organs. 



staminate (stam'e-nate) : applied to flowers which 

 have stamens but no pistils. 



sterile: free from living bacteria; also, unable to 

 reproduce. 



stigma: somewhat expanded top of a pistil of a 

 flower to which pollen is normally trans- 

 ferred. 



stimulant: something that temporarily quickens a 

 process or activity of some organ or tissue of 

 an animal or plant. 



stimulus (stim'you-luss) ; pi. stimuli: anything 

 that calls forth a response in any living thing. 



stock: in grafting, a stem to which a scion is at- 

 tached and which is its support. Also, a ma- 

 jor division in the classification of mankind, 

 such as Caucasoid, etc. 



stoma (stoh-ma) ; pi. stomata: tiny opening in the 

 epidermis of a leaf through which gases pass 

 in and out. 



strain: a variety, especially of microorganisms. Or 

 a variety of domestic animal or cultivated 

 plant produced by a breeder. 



strata (stray'ta) : parallel layers of sedimentary 

 ro(-k, each generally consisting of one kind 

 of sediment deposited continuously over a 

 long period of time. 



streptomycin (strep-toh-my'sin) : drug made 

 from soil bacteria; antibiotic. 



striated (stry'ay-ted) muscle: voluntary muscle 

 or muscle which is under the control of the 



596 



