THE VARIOUS FORMS OF PLANTS 



149 



color on tree trunks, stone houses, etc., is due to millions of these little 

 plants. 



Diatoms. — These plants arc found in vast numbers living on the mud 

 or stones at the bottom of small streams. The plant body is inclosed in 

 a eeU wall composed largely of silica. Many of 

 the diatoms are free-swimming. They compose 

 a large percentage of the living organisms found 

 near the ocean's siu-face. 



Diatoms are found as fossils, and make up a large 

 proportion of many rocks. The siliceous skeletons 

 in such rocks are of commercial importance, the 

 rock forming a basis for polishing powders. 



Various forms of Di- 

 atoms. 



Fungi, Parasites, and Saprophytes. — The 

 thallus plants may be grouped in two great 

 divisions : the Algce, water-loving thallophyics 

 containing chlorophijU, and the Fungi, thallus 

 plants li'hich do not contain chlorophyll. As a 

 direct result of the lack of chlorophyll in the 

 cells, the fungi are unable to make their own 

 food. They must obtain food from otlior plants 

 or animals. So)ne take up tlieir^ abxide^Mpon-Uving .plants or ani- 

 mals (in which case they are called parasites); others obtain their food 

 from some dead organic matter. The latter are called saprophytes. 



The above facts make the 

 group of the fungi of immense 

 economic importance to man. 



Mold (Rhizopns nigricans). — 

 One of the most common of all 

 our fungi is tlie black mold 

 which appears growing upon 

 bread, cake, and other organic 

 substances under certain con- 

 ditions of temperature and 

 moisture. 



The tangled mass of threads which cover the bread is called the 

 mycelium, each thread being called a hypha. Many of the hyphce 

 are prolonged into tiny upright threads, bearing at the top a httle 

 ball. With the low power of the microscope each of these struc- 

 tures is seen to contain many tiny bodies called spores. These 



Bread mold : r, rhizoids ; s, sporangium. 



