IRREGULAR NUTRITION 221 



the living animal, but feed upon its dead body. Lastly, there is a 

 condition known as Symbiosis, where two organisms exist together in 

 special association : this may be regarded as a kind of parasitism 

 in which the parasite is held in check, and a state of balance arises. 



There is one fact which is common to all these irregular methods of 

 nutrition. Contact with the source of supply is necessary for the 

 establishment of any of these dependent conditions. The circum- 

 stances of a crowded vegetation naturally favour this. The matted 

 roots of any sod give the opportunity for root-parasitism, such as 

 is seen in the Yellow Rattle ; the close contact of climbing plants 

 with their support offers facilities for stem- parasitism, as in the Dodder. 

 The various types of Algae attached to the submerged surfaces of 

 water-plants, or it may be actually growing in the intercellular spaces 

 of aerial parts, are common examples of close contact. This state 

 has probably led on from mere association to that physiological 

 dependence which is seen in certain Fungi. The decaying remains of 

 a crowded vegetation persist for a long time as humus or leaf-mould, 

 which itself supplies the most common source for saprophytic nourish- 

 ment. Whether the frequency of these phenomena is to be explained 

 by the advantage which the dependent organism gains by securing its 

 nourishment " ready-made," or whether there is another explanation, 

 cannot be decided on the evidence so far available. 



Irregular nutrition is not restricted to any one Family or Group of 

 Plants, but it has become the leading character of some of them. The 

 Fungi are the chief examples of it. But as they are very highly 

 specialised in this relation, while the fossil history shows that irregular 

 nutrition was established very early in them, they will be held over for 

 special study in later chapters (Chaps. XXIV. to XXVIIL). For the 

 present, the illustrations will be taken from the Flowering Plants. Some 

 families of them appear to be specially prone to physiological depen- 

 dence. For instance, the Loranthaceae, and Orobancheae. In other 

 cases isolated genera have adopted the habit ; for instance, the 

 Dodder among the Convolvulaceae, or Cassytha among the Laurels. 

 Such facts lead to the conclusion that irregular nutrition among 

 Flowering Plants is a relatively late and sporadic departure from the 

 state of nutrition characteristic of the Green Plant. 



Partial Parasites. 

 Certain plants which have adopted a parasitic habit still retain 

 their chlorophyll, though their colour is apt to be yellowish rather 

 than the full green. They are thus able to carry on photosynthesis, 



