384 



PRACTICAL BOTANY 



pushed underground. The upper portion of the stem, if it en- 

 counters a twig or small plant, quickly winds about it and 

 sends sucking roots or haustoria into the tissues of the host. 

 By means of these it draws from the host enough plant food 

 to develop the dodder plant until it flowers and seeds. If it 

 encounters no suitable host plant in the 

 course of four or five weeks, the seed- 

 ling dies. 



Some extraordinary flowering para- 

 sites develop scarcely any stem, but con- 

 sist mainly of haustoria and an immense 

 flower. Rafflesia, the most remarkable 

 of these, grows upon the roots of vines 

 which run along the surface of the 

 ground. The buds of the largest species 

 of Rafflesia on emerging from the bark 

 of the host are about as large as walnuts. 

 They finally increase in size until they 

 become very large and cabbage-like, after 

 which each bud opens into a fleshy, ill- 

 smelling flower forty inches in diameter, 

 closely attached by its haustoria to the 

 root of the host, which looks as if it bore 

 flowers of its own. 

 353. Damage inflicted by parasites. So much water and 

 plant food is taken from the host by many parasites that they 

 may cause serious injury to cultivated plants and to forest 

 trees. The flax dodder and the clover dodder often do great 

 damage to crops in this country and in Europe, and another 

 species 1 is sometimes troublesome in fields of alfalfa. The 

 American mistletoe is so injurious to dicotyledonous trees in 

 the Southwestern States that it often has to be cut off from 

 the trees to enable them to thrive. The European mistletoe 

 causes much damage to apple trees in northern France and 

 in the Tyrol. 



1 Ouscuta arvensis. 



FIG. 310. A piece of fir 



wood penetrated by the 



roots of the European 



mistletoe 



After Kerner 



