INJURIES INDUCED BY PLANTS 33 



to protect itself against being overgrown. The root usually 

 continues to grow in thickness for eight years, though it occa- 

 sionally ceases to grow in four, when it is enveloped in the adjoin- 

 ing wood owing to the formation of callus.* Thus, while it still 

 continues to grow at the point, those portions of a greater age 

 than eight years lie embedded in the wood, but they remain 

 capable of performing their functions perfectly and of taking 

 in food-substances, so long as they are not in the region of the 

 duramen (heart-wood), where water is no longer in motion. Even 

 then, however, nutriment may still be furnished to the Loranthus. 

 Here and there processes similar to medullary rays run from 

 the roots enveloped in the wood to the cortex, and at this point 

 root-shoots may be formed from adventitious buds, although 

 this occurs but seldom. 



The gnarled swelling which forms on an oak-branch attacked 

 by Loranthus is a very striking object. While the upper part 

 of the branch ultimately dies, the rugged protuberance increases 

 in thickness, and envelops the whole of the lower part of the 

 Loranthus along with its branches. The part of the oak-branch 

 that bears the swelling also increases in thickness, although it 

 possesses no leaves of its own, and there can be no doubt that 

 the products of assimilation of the parasite serve both for its 

 own nourishment and for that of the host-plant. 



As it is not expedient to shoot the thrushes even for the 

 purpose of preventing the spread of the seeds of Loranthus y 

 we must, in this case also, minimise the evil by cutting off 

 branches which are infested by the parasite. 



Although the Dodders, Cuscutece^ which are true parasites 

 destitute of chlorophyll, are for the most part injurious only 

 to herbs, they are still met with on woody plants with 

 sufficient frequency to merit a short description in this 

 treatise. Their seeds germinate in spring on the ground. 

 The young plant perishes unless its long thread-like stem 



* [Callus is the cushion-like mass of growing tissue which arises at the 

 edges of a wound and eventually covers over (occludes) the damaged surfaces 

 see Part II. under the discussion of wounds, &c. ED.] 



1 See Sorauer's Handbuch, 2nd Edition, Part II., pp. 32 48. v. Solms- 

 Laubach, Ueber den Bau und die Entwicklung parasitischer Phanerogamen. 

 Pringsheim's/<2^r. vol. iv. 



D 



