g] VITAL IMPORTANCE TO MANKIND 279 



foods from simple beginnings. For they afford the ukimate source 

 of sustenance for most of our Fishes, Crustaceans, and other ' sea 

 food ', help form Food Yeast, and give us such useful products as 

 limestones and the diatomaceous earth {see Chapter II) which is 

 widely employed in toothpastes, abrasives, filters, and so forth. 

 On the negative side. Algae may be a nuisance in clogging and scum- 

 ming and even poisoning fresh waters. 



Among miscellaneous items introduced largely by higher plants 

 are the various animal litters and ' farmyard ' manures that form 

 such an important part of the farmer's microcosm ; there are also 

 the ' green ' manures used, for example, in crop rotation, and the 

 organic mulches, etc., that are employed to conserve soil moisture 

 and improve soil texture. 



Yet another important product of higher plants is cork, obtained 

 principally from the Cork Oak, a tree native to the Mediterranean 

 region. Commercial cork consists of the outer bark of the tree and 

 can be removed every few years without injury to the tree as it 

 grows. Being exceedingly light, compressible but resilient, a low 

 conductor of heat and sound, and above all resistant to the passage 

 of moisture, it has numerous uses in industry, either in its natural 

 form or after being moulded as ' composition ' cork. Among the 

 more familiar of these uses are : as stoppers, corkboard, tips of 

 cigarettes, and handles of various kinds, in mooring-buoys, lifebelts 

 and life-jackets, footwear, tropical helmets, and various sporting 

 equipment, and in linoleum and linotiles. 



It should also be recalled that, as implied earlier, green plants 

 enable us to breathe by returning oxygen to the air during photo- 

 synthesis. They are also the primary source of most vitamins, 

 without which we would expire from a combination of deficiency 

 diseases. And all the time these same plants afford valuable research 

 materials on which many of the greatest scientific discoveries have 

 been made and important studies continue practically throughout the 

 world. 



Some Nuisances 



The significance of various lower plants, particularly, in causing 

 diseases of animals and other plants has already been referred to. 

 Thus, such' human diseases as tuberculosis, brucellosis, tetanus, 

 typhoid and some other fevers, plague, cholera, diphtheria and many 

 more, are all due to Bacteria, while the Potato and Chestnut Blights 



