OTHER METHODS OF OBTAINING FOOD 199 
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 
occasionally 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. Theadvantage of the 
alliance is on. the side of the Mistletoe, which derives a 
great part of its nourishment from the host. It possesses 
evergreen leaves, however,- which serve for the construc- 
tion 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 
dependence of one form, the parasite, upon the other, the 
host. As in the case of the insectivorous plants, there are 
members of this class which are provided with a chlorophyll 
apparatus, and which are therefore indebted to their hosts for 
protein substances only, or perhaps also for certain of their 
ash constituents. As these almost without exception fasten 
