610 The Outline of Science 



But those fungi, the parasites, which feed on the living plant, 

 are often very harmful, sapping the substance of their hosts and 

 killing them by the poisons they manufacture. They are the chief 

 causes of plant disease, and we get a glimpse of their fatality 

 when we see a potato field reduced by blight, in a week or two, 

 to a mass of rotting and offensive hauhns. 



Lichens as Double Plants 



The familiar lichens, which cover such unpromising places 

 as stones and tree- trunks, are double plants, composed of a fungus 

 and an alga growing together in a partnership so intimate that the 

 result maintains a specific form. Without the aid of the micro- 

 scope their dual nature would never have been discovered, and it 

 was not held definitely proved until the two partners had been sep- 

 arated by a delicate technique, grown in isolation, and then made 

 to reconstitute the lichen on being brought together once more. 



Flowering Parasites and Saprophytes 



Among the flowering plants there are some very interesting 

 saprophytes and parasites. Thus, among British plants the bird's 

 nest, the bird's-nest orchis, and the coral-root are saprophytes, 

 the first belonging to the heath family, the other two being orchids. 

 All three grow habitually in the leaf -mould of woods, a soil rich in 

 decaying organic matter. Though the details of the relationship 

 are not fully known, it seems that a symbiotic fungus which in- 

 habits the roots of these plants play some part in making the food 

 in the soil available. In none of the three is there any chlorophyll ; 

 the bird's-nest is cream-coloured, the two others are dirty brown. 

 Even the leaves are much reduced, being represented only by 

 pointed scales. 



More familiar are parasites, such as the mistletoe, the dodder, 

 and the broomrapes. The mistletoe is only partially dependent on 

 its host, the apple or the fir, for it has green leaves, and can 

 manufacture carbohydrates for itself. 



