90 PLANT PHYSIOLOGY 



Its ability to develop chlorophyll in times of need suggests 

 two hypotheses : that it has only recently abandoned the in- 

 dependent habits still followed by the other members of the 

 family Convolvulacese, and that it has done so in much 

 the same way as the mistletoes, though the latter have 

 progressed by no means so far toward permanent para- 

 sitism. 



It is hardly necessary to discuss whether the host is bene- 

 fited by the encircling dodder, for the dodder is an annual, 

 dying soon after ripening its fruits, into which it removes 

 the greater part of the nutrient substances contained in and 

 composing its body. Thus it leaves little or nothing for its 

 host to absorb and feed upon. When it attacks shrubby 

 perennials e. g. willows its effects are only impoverishing 

 and debilitating; when it successfully attacks annuals, it 

 exhausts and kills them. European growers of flax and 

 clover find the dodder one of the most destructive enemies 

 of their crops. The dodder is, then, a permanent parasite, 

 the parasitism of which is complete, however, only when the 

 host can supply it with all needed foods. 



The most intimate associations between parasite and 

 host, and the most complete dependence of a parasite upon 

 its host, are found among the fungi and bacteria parasitic 

 upon higher organisms. In these associations the parasite 

 not only sends root-like absorbing organs into the host, 

 but in many cases it is very completely enclosed within the 

 host.* The parasitic fungi and bacteria, always wholly 

 devoid of chlorophyll, are entirely dependent upon their 

 hosts for both kinds of food, non-nitrogenous and nitro- 

 genous. These foods they work over, assimilate, incorpo- 

 rate and consume, in their own w r ays, but the elaborated 

 foods they must have. These, then, are examples of per- 

 manent and complete parasitism. About the nature of the 

 association between green plants and the fungi which cause 

 disease in them, e. g. grape-mildew, potato-rot, wheat-rust, 

 etc. there cannot be the least question : the host gains 



* This last is not without parallel among phanerogamic parasites, for 

 Rafflesia, Brugmnnsia,, and our own Arceuthobium (Razoumowskia) are 

 more or less completely imbedded in the tissues of the host. 



