THE EVOLUTION OF THE PLANT BODT 



47 



multicellular filaments make up the bulk of the mold, but these 

 are dependent upon root-like tufts of absorbing filaments which 

 penetrate the substratum and absorb organic materials (fig. 23). 

 The mold carries on the same type of existence as does the bac- 

 terial cell, with the added complications associated with a multi- 

 cellular body. A mold growing on the surface of a piece of bread is 

 continually absorbing carbohydrate and other organic material 

 through its rhizoids; this food passes slowly through cell after 

 cell, eventually reaching every 

 cell of the body. With ample 

 food, the newly formed proto- 

 plasm stimulates cell division, 

 so that growth of the mold is 

 the result of an increase in the 

 number of cells. 



Mushrooms attain greater 

 size than molds, reaching the 

 greatest size of any heterotrophic 

 plant. But even with this greater 

 size, there is not much more 

 division of labor in the plant 

 body, and little tissue speciali- 

 zation. A common field mush- 

 room (fig. 24) consists of an 

 almost entirely subterranean 

 maintenance portion; the above- 

 ground and familiar structure with stalk and cap is in reality the 

 reproductive outgrowth from the subterranean vegetative body. 

 This body is a mass of unspecialized cells, of a filamentous 

 nature and similar to that of the mold. From these, absorptive 

 filaments penetrate the humus or other source of organic 

 material. Metabolic activity, involving food getting and utili- 

 zation, is the same as that taking place in the mold. 



Even with their lack of complex maintenance tissues or 

 organs, the heterotrophic plants have become numerous and of 

 great economic importance. It is said that for every two self- 

 sustaining green plants there is a saprophytic or parasitic species 

 depending upon them for sustenance. Since these destructive 



Fig. 23. — Division of labor in a 

 mold plant has resulted in a mass of 

 colorless vegetative filaments with 

 root-like absorbing outgrowths. 



