1 88 BOTANY OF THE LIVING PLANT 



as in the Lichens. This has been described as Symbiosis, or Mutualism. 

 There being thus a wide range of variety in these states of dependence, 

 the subject will be best approached through the description of actual 

 examples rather than by general statements. 



There is, however, one fact which is common to them all. Contact 

 with the source of supply is necessary for the establishment of any of 

 these conditions. A crowded vegetation naturally favours 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 growth of various Algae attached to the submerged surfaces of 

 water-plants, or it may be actually 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. The advantage which the dependent organism gains, by get- 

 ting nourishment without needing to manufacture it for itself, offers 

 a ready explanation of the frequency of these phenomena. 



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, w r hile the fossil history shows that irregular 

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

 for special study in later chapters (XXVI.-XXIX.). 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 advance upon the 

 primitive 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 Photo-Synthesis, 

 and to produce at least part of their own nourishment. They are only 



