598 - Heredity and Evolution 



Fig. 31-3. One of the golden-brown algae (Chrysophyta), the diatom, Pinnularia. Note 

 the finely etched, glassy cell wall and the overlapping halves of the "capsule." These 

 features are common to all diatoms. (From The Plant World, by Fuller and Carothers. 

 Holt, Rinehart and Winston, Inc.) 



is formed like a medicinal capsule — with 

 tu<o symmetrical slightly overlapping halves 

 (Fig. 31-3). Moreover, the capsule is finely 

 etched with dots and markings of such ex- 

 quisite delicacy that diatoms are often used 

 by microscopists for testing the resolving 

 power of high quality lenses. 



Typical nuclei and chloroplasts are pres- 

 ent (Fig. 31-3). However, in addition to 

 chlorophyll, the chrysophytes possess a brown 

 pigment, fucoxanthin, which is likewise 

 found in the brown algae (p. 599) and in 

 the (lame algae (p. 598). The intracellular 

 storage product is a fluid lipid, generally re- 

 ferred to as oil, rather than solid granules of 

 a carbohydrate nature. 



Diatoms occur in enormous abundance and 

 variety in most salt and fresh waters. A 

 cubic loot of ocean water frequently will 

 contain well over a million specimens. In 

 fact, the diatoms, together with the dinofla- 

 gellates (p. 599), are sometimes referred to 

 as the "grass of the sea," because they repre- 

 sent such an important food base for fish 

 and other marine animals. By dint of num- 

 bers these organisms appear to be responsible 

 directly and indirectly for the synthesis of 

 considerably more than half of all existing 

 organic matter (p. 158). And in past ages, 

 extending back into the early Paleozoic era, 

 diatoms nourished in equal if not greater 

 abundance. Some sedimentary deposits of 

 diatomaceous earth, which consists largely of 

 the distinctively marked, silicon-laden cap- 

 sules of ancient diatoms, display a depth 

 greater than 1000 feet. This material, be- 

 cause of its exceedingly fine abrasive quali- 

 ties, is used extensively in the preparation of 



dentifrices and of metal polishes. Also dia- 

 tomaceous earth, owing to its fine porosity 

 and excellent adsorptive capacity, is very 

 useful in the manufacture of insulating mate- 

 rials and of special filters. And finally, there 

 is considerable evidence that supports the 

 view that much of our present-day petroleum 

 owes its origin to the large accumulations of 

 oil in the diatoms of ancient times. 



The rich fossil record indicates that the 

 Chrysophyta originated as a distinctly sepa- 

 rate group in very early times, and that they 

 did not give rise to any of the higher plants 

 (Fig. 29-11). 



The Pyrrophytes (Flame Algae). By far 

 the most abundant and important of the 

 flame algae are the dinoflagellates, two 

 species of which are shown in Figure 31-4. 

 These curiously formed unicellular plants 

 are predominantly marine in habitat. In the 

 sea they often generate populations of tre- 

 mendous density, and thus they provide a 

 food base for a multitude of fish and other 

 marine animals (p. 589). 



Dinoflagellates display typical nuclei and 

 chloroplasts, but the cell wall takes the form 

 of a number of overlapping cellulose plates 

 (Fig. 31-1). Always there are two flagella (Fig. 

 31-4), one lying in a groove that tends to 

 encircle the cell and the other trailing pos- 

 teriorly. 



The reel color of most Dinollagellata comes 

 from the pigment fucoxanthin, which is 

 present in addition to chlorophyll. Some 

 dinoflagellates synthesize products that are 

 quite toxic to other organisms. Occasionally, 

 indeed, the fish in certain waters (for exam- 

 ple, the Gulf of Mexico) may be killed off by 



