PARASITES AND SAPROPHYTES. 83 



ena. The acid salts are the substances which are most actively concerned 

 in reddening the blue litmus paper. They do not directly aid in the corro- 

 sion phenomena. In the soil, however, where these compounds of potash, 

 phosphoric acid, etc., are which are not soluble in water, the acid salt 

 (primary acid potassium phosphate) which is most actively concerned in 

 reddening the blue litmus paper may act indirectly on these mineral sub- 

 stances, making them available for plant food. This salt soon unites with 

 certain chlorides in the soil, making among other things small quantities 

 of hydrochloric acid. 



176. NOTE. It is a general rule that plants cannot take solid food into 

 their bodies, but obtain all food in either a liquid or gaseous state. The 

 only exception to this is in the case of the plasmodia of certain Myxomy- 

 cetes (Slime Moulds), and also perhaps some of the Flagellates and other 

 very low forms, which engulf solid particles of food. It is uncertain, how- 

 ever, whether these organisms belong to the plant or animal kingdom, 

 and they probably occupy a more or less intermediate position. 



177. Action of nitrite and nitrate bacteria. Many of the higher green 

 plants prefer their nitrogenous food in the form of nitrates. (Example, 

 nitrate of soda, potassium nitrate, saltpetre.) Nitrates are constantly 



icing formed in soil by the action of certain bacteria. The nitrite bacteria 

 (Nitromonas) convert ammonia in the soil to nitrous acid (a nitrite), while 

 at this point the nitrate bacteria (Nitrobacter) convert the nitrites into 

 nitrates. The fact that this nitrification is going on constantly in soil is of 

 the utmost importance, for while commercial nitrates are often applied 

 to the soil, the nitrates are easily washed from the soil by heavy rains. 

 These nitrite and nitrate bacteria require oxygen for their activity, and 

 they are able to obtain their carbohydrates by decomposing organic matter 

 in the soil, or directly by assimilating the CO 2 in the soil, deriving the energy 

 for the assimilation of the carbon dioxide from the chemical process of 

 nitrification. This kind of carbon dioxide assimilation is called chemo- 

 synthetic assimilation. 



2. Parasites and Saprophytes. 



178. Parasites among the fungi. A parasite is an organism which 

 derives all or a part of its food directly from another living organism (its 

 host) and at the latter's expense. The larger number of plant parasites 

 are found among the fungi (rusts, smuts, mildews, etc.). (See Nutrition of 

 the Fungi, paragraph 185.) Some of these are not capable of develop- 

 ment unless upon their host, and are called obligate parasites. Others can 

 grow not only as parasites but at other times can also grow on dead organic 

 matter, and are called facultative parasites, i.e. they can choose either a 

 parasitic life or a saprophytic one. 



179. Parasites among the seed plants. Cuscuta. There are, however, 

 parasites among the seed plants; for example, the dodder (Cuscuta), para- 



