118 PLANTS AND MAN 



and delicate flavor than by the presence of nutrients in their 

 tissues, which are chiefly composed of water. The Greeks and 

 Romans prized mushrooms as food deUcacies, and through suc- 

 cessive ages various peoples have held them in high esteem. 

 Today they are the only thallus plants to be commercially culti- 

 vated for eating purposes. This is the safest method of securing 

 mushrooms, for eating wild species is dangerous unless one is sure 

 of the identification of the edible and poisonous varieties. There 

 are some two hundred edible species, the best known being the 

 puffballs, morels, truffles, chantrelles, chicken and field mush- 

 rooms. Lichens are relatively indigestible because of the bitter 

 acids they contain, but several genera are eaten by animals and 

 people of some races. Reindeer lichen is the chief food of semi- 

 arctic land mammals, and a related genus known as Iceland moss 

 contains a starchy food eaten by the inhabitants of Iceland. Of 

 historical importance is the lichen Lecanora found in northern 

 Africa and western Asia, which was the "manna" of the Israelites. 



The leafy spore plants are of negligible food value, since 

 mosses and ferns contain insufficient nutrient material to attract 

 animals or man. It is quite diff*erent, however, with the leafy 

 seed plants {Spermatophyta) . Because of their greater size and more 

 efficient maintenance organs, as well as their habit of storing 

 food in roots, stems, fruits and seeds, many of these have become 

 staple adjuncts of the human diet. 



The vegetative parts of a plant are of importance as foods, in 

 proportion to the degree in which they are used as storage organs. 

 Ordinary plant tissues, being made up of cellulose-walled cells, 

 are indigestible and of little nutrient value. Among the few 

 Gymnosperms whose vegetative parts have food value, we have 

 already noted the starchy underground stems of the sago palm 

 (see p. 101); few other species in the class are of any food impor- 

 tance. Among the Angiosperms, on the other hand, there are 

 many species with edible roots, stems or leaves. The storage of 

 food in the roots of beets, carrots, radishes, turnips and sweet 

 potatoes have made these plants useful as vegetables. In addition 

 there are many other species whose roots store food — usually 

 starch — and which have been used by various peoples as 

 foods. Digging for roots is one of the most primitive ways of 



