=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 _Myxomycetes_ (Slime Moulds), and also perhaps some of the Flagellates and other very low forms, which engulf solid particles of food. It is uncertain, however, 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 being 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 CO₂ 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 _chemosynthetic_ 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 development 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), parasitic on clover, and a great variety of other plants. There is food enough in the seed for the young plant to take root and develop a slender stem until it takes hold of its host. It then twines around the stem of its host sending wedge-shaped haustoria into the stem to obtain food. The part then in connection with the ground dies.