24- THE PLANT WORLD. 



the use of organic compounds is shared quite widely by green plants. 



The acquisition of organic food-substances is not only of great 

 value to the plant because of the energy stored up in such compounds, 

 but also from the fact that the absorption and assimilation of soil sub- 

 stance goes on almost uninterruptedly. The initial processes of 

 manufacture of organic compounds by green plants may take place 

 only in light. Any great increase in the power of absorbing organic 

 material entails changes of the most serious character in the structure 

 and physiology of the plant. These changes are most noticeable in 

 the absorbing organs, chlorophyll apparatus, and transpiratory organs. 

 The series of disturbances resulting, often changes the entire aspect 

 of the plant. The effects of saprophytic methods of nutrition are to 

 be seen among lower forms in bacteria and fungi, and Indian pipe i^Mono- 

 tropa), and Coral Root {Corallorliiza), are examples in the seed-form- 

 ing group. 



The amount of organic material in fresh waters varies between 

 .0006 per cent, and .1 per cent, under ordinary circumstances. In 

 any instance the amount available for absorption is much beyond the 

 assimilative capacity Qf the green aquatic on account of the limited 

 supply of oxygen at its disposal. The air contains 20 per cent, of this 

 element, while water holds in solution but 3 per cent, at ordinary 

 temperatures. The first step in the development of an aquatic plant 

 toward saprophytism, then, would be some adaptation for securing an 

 increased supply of oxygen, or a modification of the respiratory 

 metabolism, rather than a variation of the absorbing capacity. The 

 earlier development has been one which would render the absorbing 

 membranes most suitable for the use of mineral salts only. In more 

 recent periods the tendency to the absorption and use of organic sub- 

 stance has become strengthened in the members of all the great 

 groups of the vegetable kingdom. So far as the material at hand may 

 be interpreted, saprophytism was not shown among the seed-forming 

 plants until the present geologic period. 



Land plants have developed three general methods of saprophy- 

 tism or absorption of organic food. By one method the membranes 

 of the absorbing organs are so modified as to permit the passage of 

 high molecular complexity, in the same manner as in fungi. This en- 

 tails profound variation in the diosmotic properties of the peripheral 

 layers of the protoplasm of the outer cells of the roots, and only one 

 species is known, Wiillschlcegelia aphylla, a colorless orchid from the 

 West Indies, that has been able to attain complete saprophytism, or 

 obtain its entire food supply in this manner. 



By a second method, plants entrap or receive the bodies of ani- 

 mals and plants in organs developed from the shoot, and absorb the 



