768 



ECOLOGY 



The chemotropic theory of parasitism would regard obligate parasites as forms 

 that react only to substances that are found in living plants, and univorous parasites 

 as forms that react only to substances that are found in a single host species. This 

 theory is not universally accepted, but no more tenable hypothesis has been sug- 

 gested. In all theories of parasitism, especially in those that attempt to account 

 for the phenomena of heteroecism and specialization, it is difficult to explain the 

 fixation of new habits, inasmuch as there is no evidence that variability is ever lost. 

 Hence appeal often is made to mutation as the decisive element in determining the 

 origin of the forms. 



Immunity and related phenomena. Most cultivated plants are sub- 

 ject to bacterial or fungal diseases. In many species certain varieties 

 are particularly subject or predisposed to infection, whereas other 

 varieties are disease-resistant or immune. Few phenomena are 



more puzzling than are those 

 of immunity; in many cases 

 it may be attributed to the ab- 

 sence of substances that induce 

 prochemotropic reactions, or 

 to the presence of substances 

 which either induce apochemo- 

 tropic reactions or neutralize 

 the enzyms secreted by the 

 fungus. In addition to factors 

 of a chemotropic nature, im- 

 munity has been referred to 

 the presence of an impene- 

 trable epidermis, to the se- 

 cretion of antitoxins, and to 

 the absence of the proper 

 food for the parasite. Some 

 forms, as Penicillium italicum 

 and Pseudomonas destructans, 

 secrete toxins poisonous to 

 themselves, thus making the 

 host plant immune through 

 their own activity. Immunity 

 FIG. 108 1. A portion of a shoot of a dodder of this character is experi- 



(Cuscuta Gronovii), a sinistrorse twining para- ence( J commonly by many 



site; note the haustoria (h) which penetrate the 



tissues of the host plant (); the leaves (5) are P lants and animals that have 



minute scales ;/, flowers. been subject to certain bac- 



