18 REACTION OF HOST TO PARASITIC ATTACK. 



utilize the water which would otherwise have ascended to 

 higher parts of the branch-system (Fig. 3). It must indeed 

 be assumed that the latter are preyed on by the hypertrophied 

 parts and give up plastic material, which they would otherwise 

 have utilized themselves or stored up as reserve material. 

 On branches attacked by mistletoe and other phanerogamous 

 parasities, it can easily be observed, particularly on broad- 

 leaved trees, that a supporting branch grows vigorously in the 

 parts under the influence of the root-system of the parasite, 

 whereas the distal parts of the same branch-system remain 

 stunted and finally die. The mistletoe- bush thus comes to form 

 the termination of the supporting branch. If, in consequence 

 of this, the branch ceases to produce the leaves necessary in 

 preparing food for it, then like every other leafless branch it 

 dies. Such branches carrying leaves of the mistletoe alone may 

 frequently be found on firs, pines, and broad-leaved trees ; 

 even whole tree-summits have been seen on the silver fir with 

 every branch terminated by a mistletoe-tuft, not unlike some 

 huge candelabrum. 



In a similar manner a witches' broom, developed from a 

 lateral bud, exhibits throughout an increased growth, while the 

 branch supporting it remains thin and dies from the insertion 

 of the broom outwards. So also in attacks of Gymnosporangium, 

 on juniper it may be observed that the parts attacked have 

 their growth much accelerated and many of their dormant buds 

 developed, while the distal parts of the same branch die off. 

 In all such cases it is quite probable that, as the distal parts 

 die back, any food material which they may contain finds its 

 way into the hypertrophied region. 



C. Shortening of Life. 



Many fungi inhabit a plant without disturbing the develop- 

 ment of any part or causing immediate death, yet with such 

 effect that the vegetative period of the organ in question 

 terminates earlier than normally. 



A very striking example of this is presented by the needles 

 of silver fir on the witches' brooms caused by Aecidium 

 elatinum. The needles normally vegetate for several years, 

 but when influenced by this parasite they live only a single 

 season. So also needles of spruce attacked by Aecidium cor- 



