CHAPTER XXIV 



PLANTS AS DISEASE PRODUCERS, EPHIPHYTOTISM, 

 PROPHYLAXIS 



Vegetal Agents of Disease. — The plants which are known to 

 be injurious to other plants fall naturally into two large groups, namely, 

 the Phanerogamic and the Cryptogamic. The latter includes injurious 

 algae, slime moulds, bacteria and fungi. 



The phanerogamic parasites belong to four families of plants. 

 Their morphology and physiology is fairly well known, so that in their 

 discussion, we are entering well-trodden fields of investigation. 



The flowering plants, which lead a partially or wholly dependent 

 life upon a host plant, may be considered as belonging to two distinct 

 groups: the green parasites and the chlorophylless parasites. The 

 plants of the first group illustrate by gradations how the conditions of 

 life of the second group have arisen. The seeds of the first series of 

 green parasites begin their growth in the soil and there develop into 

 seedlings with cotyledons and root system, without any connection 

 with a host plant. The root branches supplied with suckers then 

 become attached to the roots or underground stems of other plants. 

 About one hundred plants of the sandalwood family, Santalace^, 

 belong to this series, including the true sandalwood, Santalum album 

 of India, where its roots live attached to the roots of a species of Acacia 

 leucophcBa and Pride of India, Melia azidarachta^. 



The bastard toad-flax of Europe, Thesium alpinum, is another 

 member of this family. It develops relatively large suckers, which 

 become attached to the roots of other plants. These suckers are con- 

 stricted near their point of insertion. The swollen part spreads itself 

 over the root of the host as a plastic mass, while the central cores per- 

 forate the root and grow into the wood of the host where they spread 

 out. Comandra umbellata is a santalaceous parasite found in the pine- 



" Wilson, C. C: Sandalwood. Indian Forester, xli: 248, August, 1915. 

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