138 OUTLINES OF PLANT LIFE. 



184. Parasites obtain their food either by growing upon 

 the surface of the host and thrusting into its interior absorb- 

 ing organs ; or by growing wholly in the interior of the host, 

 breaking out to its surface only to form reproductive bodies. 



Parasites may work little apparent harm, or they may bring 

 about local disease and death of the host. Their mode of 

 obtaining food is not essentially different from that of sapro- 

 phytes. 'J'hey either digest solid foods, or absorb liquid 

 foods, prepared by the host for its own use. Among the 

 green plants there are some partial parasites, such as the 

 mistletoe, which seem to obtain from their host chiefly the 

 water and salts which they have absorbed. These materials 

 they themselves elaborate into food. (See further *\ 370.) 



D. Nutrition of green plants. 



185. Raw materials. — In order that the green plants may 

 be able to manufacture their food, they require certain raw 

 materials, which are obtained from the water and air. The 

 water is always a weak watery solution of various mineral salts. 

 From the air (or the water in the case of submerged plants) 

 they absorb a gas, carbon dioxid. 



186. Salts absorbed. — Along with the water which is 

 taken into the plant go various amounts of dissolved material, 

 a considerable portion of which consists of mineral salts. 

 When plants grow in humus, or in water or soils containing 

 organic matter, a variable amount of carbon compounds suited 

 for food may be dissolved by the water and be taken up by the 

 plant. To this extent the plant will live as a saprophyte, 

 and no doubt many field and garden plants have been bred to 

 require this sort of life. 



Among the mineral salts the most important are the salts of potassium, 

 magnesium, calcium, and iron, which are present in all soils, in greater 

 or less quantity, and are dissolved in surface waters. In the same way 



