OTHEE METHODS OF OBTAINING FOOD 205 



rise subaerial stems from ten to twenty centimetres high, 

 bearing succulent membranous leaves. From the rhizome 

 are given off crowded masses of roots which are covered 

 with a mycorhizal mycelium, and are embedded in humus. 

 There being no chlorophyll apparatus, Monotropa is de- 

 pendent entirely on the mycorhiza for its nourishment. 

 The latter is entirely saprophytic. We have here a curious 

 case of the complete dependence of a higher plant upon 

 a more lowly one. 



A complete symbiosis between two green plants is occa- 

 sionally met with. A good instance is afforded by the 

 Mistletoe and the plants upon which it grows, usually 

 either the Poplar, the Silver Fir, or the Apple-tree. The 

 seed of the Mistletoe is left by a bird upon a branch of 

 one of these trees, and under appropriate conditions it 

 germinates. The root of the seedling penetrates into the 

 bark of the tree and grows inwards till it reaches the wood. 

 It makes its way no further, but maintains its position 

 there, and as the branch gradually thickens by the activity 

 of its cambium, the intruding root is by degrees impacted 

 in the secondary wood, its own growth preventing its being 

 cut off and buried by the latter. The root branches in the 

 substance of the tree, and the secondary roots make their 

 way along in the bast, growing parallel with the exterior. 

 These branches also put out small vertical outgrowths, 

 which make their way to the wood just as the primary 

 root did. A very complete fusion of the tissues of the two 

 plants is thus ultimately arrived at. The advantage of the 

 alliance is on the side of the Mistletoe, which derives a great 

 part of its nourishment from the host. It 'possesses ever- 

 green leaves, however, which serve for the construction 

 of carbohydrates, and as it manufactures these during the 

 winter, when the host plant has no leaves, the latter is able 

 to benefit in its turn during that season. 



Passing on to notice the association of two organisms 

 which is known by the name of antagonistic symbiosis or 

 parasitism, we find various degrees of completeness in the 



