THE EVOLUTION OF THE PLANT BODY 45 



trophic organisms. Since there is no dependence upon sunUght, 

 they frequently abound in shaded and dark habitats where photo- 

 synthetic plants could not live. Such plants have no leaves, stems 

 or roots; the majority are microscopic or small in size, with little 

 tissue specialization and hence belong to the thallus plant group. 

 Their maintenance activities center about the securing of food, 

 and as a result the plant body is designed for the absorption of 

 organic materials from the surroundings. 



The absorption of food is brought about in one of two ways. A 

 SAPROPHYTE is a heterotrophic plant which secures its organic 

 material from the bodies of dead organisms or their excreta, or 

 from the debris of plant life known as humus. Saprophytes are 

 often scavengers and usually not detrimental to man. In some 

 cases, as among the decay bacteria, the saprophytes are actually 

 of great value in the continual cycle of food elements in nature. 

 Parasites on the other hand absorb the organic material directly 

 from the bodies of living animals and plants; this often disturbs 

 the normal activities of the host and causes its death. Plant 

 parasites are man's deadliest enemies, causing the majority of 

 human diseases and diseases of animals, as well as plant diseases 

 which take an expensive annual toll among our crop plants. 

 This aspect of the relationship of plants to man is more fully 

 treated in Chapters 24 and 25. 



Other maintenance activities of saprophytes and parasites 

 are similar to those of the green plants; respiration, excretion, 

 growth, protection against unfavorable factors in the environ- 

 ment. A consideration of three body types — bacteria, molds and 

 mushrooms — chosen from the thousands of heterotrophic plants 

 will suffice to show the range in body complexity. 



Bacteria are the smallest organisms known to man, some 

 species being only 1/250,000 of an inch in length. They are uni- 

 cellular colorless plants found in liquid or semi-liquid environ- 

 ments. In contrast to the desmid cell, the bacterial organism has 

 an exceedingly simple structure (fig. 22). A thin cell wall may at 

 times develop a thick gelatinous sheath or capsule for protective 

 purposes; within this, the protoplasm is not clearly organized 

 into nucleus and cytoplasm, the chromatin material being scat- 

 tered throughout the cell. Few cytoplasmic structures are dis- 



