214 VEGETABLE PHYSIOLOGY 



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 

 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 

 proteid substances only, or perhaps also for certain of their 

 ash constituents. As these almost without exception fasten 

 themselves upon the roots of the host plant, they are 

 frequently spoken of as root-parasites. From their general 

 structure and their relationship to the host plant, they 

 evidently have much in common with the Mistletoe, and 

 it is not very easy to distinguish between their semi- 

 parasitism and the symbiosis of the latter with the trees 

 on which it lives. They are, however, usually herbaceous 

 forms, and can - therefore be of no use to the host plant 

 in the winter. Moreover, most of them ultimately destroy 

 the root on which they have fastened. 



These root-parasites are mainly members of the Scroph- 

 ulariacece or the Santalacece. As a rule, they are 

 herbaceous annuals, though there are some perennial 



