THE LARGE GROUP OF THE SMALLEST ()H(;.\MSMs 



i«.:{ 



the protoplasmic bridge, and unites with the larger niicroiiuclcus of 

 the cell left behind. The two conjugating cells now separate, and the 

 newly fused nucleus, composed of a male and female microinu'lcus, 

 is left in each cell. Then a series of divisions of this nucleus takes 

 place until eight nuclei are formed, four of which become macro- and 

 four micronuclei. Three of the micronuclei next disintegrate, leaving 

 the cell with four macro- and one niicionucleus. The latter divides 

 again and with it the cell, so that two cells result, each witli a inicro- 

 and two macronuclei. A second division leaves the daughter cells 

 each with a single macro- and micronucleus, which, thus rejuvenateil. 

 start off on a series of several hundred cell divisions until another 

 period of old age comes on, when conjugation or endomixis is repetited. 



Diatoms 



These beautiful microscopic plants, sometimes called "jewels of the 

 plant world," are among the most numerous of the one-celled plants. 

 Over 2000 species have been identified and named. They form one 

 of the most abundant components of plankton in 

 both fresh and salt water, and are also found in 

 damp earth and on moist rocks, where they may 

 occur singly or massed together in groujis. Certain 

 species stick together because of a gelatinous ma- 

 terial which they secrete. Some diatoms move 

 with a slow gliding motion when they are in con- 

 tact with solid objects, although lacking visil)le 

 organs of locomotion. They secrete a glasslike 

 shell exquisitely marked by tiny ridges and rows 

 of extremely minute holes. 



Diatoms have been, and still are, among the 

 most abundant of li\ing organisms. So abundant 

 were they in past ages that large deposits of their 

 shells exist in the form of diatomaceous earth. 

 In California, there are deposits of diatomaceous 

 earth lying hundreds of feet thick over an area of 

 many square miles, while the floor of the ocean is 

 covered with ooze made up of skeletons of diatoms, 

 which after death sink to the bottom of the water. 

 This diatomaceous material is used as a basis for i^ohshnig lu.wders 

 in the manufacture of bacteriological filters, and of certain kinds o\ 

 porcelains and glass. 



The (lialoiii \(i- 

 vinild («) Niilxt-sidf, 

 {h^ ;:inilc side. sIkiw- 

 inf.' IIk' rcliilion of 

 tlic\;iK('s. ThiMiii- 

 clfiis iiiid the two 

 rililmiilikc chlorn- 

 pliislsitrc iiol sliowii. 

 (After Plil/«T.i 



