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TESTA

1894

TETRASPORE

electricity and magnetism. On coming to the United States in 1884, he for a time was in the Edison works at Orange, N. J., but afterwards became electrician in the Tesla Electric-Light Company, and sstablished a laboratory at New York for independent research. He is the inventor of the modern principle of the rotary magnetic field embodied in the apparatus used in the transmission of power from Niagara Falls; of new forms of dynamos, transformers, [induction-coils, condensers, arc and incandescent lamps; and of the oscillator combining steam engine and dynamo. His researches in electrical oscillation have created a new field of electrical investigation; while he has made many startling innovations and inventions in using currents of high tension. He continues to reside in New York.

Tes'ta, the seed-coat, which usually is hard and bony. The testa is developed by the tissue of the integument of the ovule. Sometimes it develops into two very distinct layers, an outer fleshy layer and an inner hard one. Often it is variously marked and sculptured on the surface, enabling one to recognize the plant from which it has come. In fact, the color and markings of the testa are very important guides in the classification of many plants. It is the testa also which develops the various tufts of hairs and wings, which are often found on seeds and aid their dispersal by the wind. It sometimes develops hooks and spines, as grappling appendages, to aid in dispersal by animals,

Test'ing=Machine, a machine for testing materials. Testing-machines are arranged so as to apply measured forces to a sample of the material and to observe the corresponding deformation or breaking of the sample. The common tests are stretching, crushing, bending and tension. It is usual to apply the force either by a system of levers or by means of a hydraulic press. The forces are usually measured by a scale-beam. When large samples are to be tested, as long, heavy beams, the machine must be very large. The largest and finest testing-machine ever constructed is that at the Watertown arsenal, belonging to the United States government. It can exert a force of 450 tons, and yet it is extremely sensitive. At the time of its trial a rod of iron five inches in diameter was broken with a meas-

ured force of 722,000 pounds and immediately afterward a horse-hair was broken with a measured force of one pound. For loads of 200 pounds it is sensitive to less than one two-millionth of the whole. Testing-machines have so many special forms and appliances that special treatises must be consulted for details. See Unwin's Materials of Construction.

AN AMERICAN TESTING-MACHINE

Tetrad (tet'rad) (in plants). In most of the higher plants each mother-cell organizes four asexual spores within itself. This group of four is called a tetrad. These four sister-spores cling together for a time, and sometimes always, but generally they soon separate from one another. For example, if a very young anther be sectioned and examined under the microscope, each spore-bearing cell (mother-cell) within it will be seen to con-

A MOTHER-CELL  ORGANIZING   FOUR   SPORES   WITHIN   ITSELF

(A TETRAD).

tain just four forming pollen-grains. This tetrad formation of asexual spores is almost without exception rrom mosses to seed-plants. Tet'raspore (in plants), the asexual spore of red algae. It is so named because each