68 DISEASES OF TROPICAL PLANTS en. 



Other algae form patches ou leaves, twigs, and 

 branches of the plants and live partially as parasites or 

 in such a manner as to seriously interfere with the 

 function of the plants, as in the case of Ceplialeurus 

 mycoidea, Karsten, of the tea (page 174). 



Seed Plants. A number of species of seed plants 

 have become partially or entirely parasitic upon other 

 plants which they injure and sometimes destroy. 

 Among the most widely distributed of these parasitic 

 plants are the dodders or love vines (Cuscuta). The 

 seeds of the dodder germinate in the soil, producing a 

 colourless vine which, upon coming in contact with the 

 proper host, twines round and becomes attached to it 

 by haustorial growths. It then loses its connection 

 with the soil and continues its existence at the expense 

 of the host plant. 



The mistletoes (Fig. 36) are semiparasitic and are 

 widely distributed throughout most parts of the world. 

 The germinating seeds of these plants send their root- 

 like haustorial growths directly into the tissues of the 

 host plant. These plants have well-formed leaves, con- 

 tain chlorophyll and so are not entirely dependent upon 

 the host plant for nourishment. However, they take a 

 great deal of water and possibly some salts from the 

 host plant, and cause dwarfed growths and malforma- 

 tions, and frequently cause the death of a part or all of 

 the host plant. 



The broom rapes are widely distributed over the 

 earth, and many species are known to live in tropical 

 countries. Some of them attack the agricultural crops 

 and do considerable damage. The seeds do not 

 germinate, in some species at least, unless they come in 

 contact with the proper hosts. A definite union is 

 formed between the host and parasite which can only be 

 seen by removing the soil from the roots. Not all the 

 roots are attached to the host, but serve the ordinary 

 functions of nonparasitic plants (page 159). 



Bacteria. These interesting little organisms are 

 the causes of many diseases in plants as well as in 



