382 



PHYSIOLOGY 



FiG. 652. Section of stem penetrated by haustorium 

 (h) of dodder (Cuscuta). From PART III. (For ex- 

 planation of letters, see fig. 1082.) 



solving the wall by some enzyme (see digestion, p. 399), or it may send 



into them short branches, called haustoria (fig. 651; see also figs. 1079, 



1080, Part III), through 

 which the food enters 

 the parasite. A vascu- 

 lar parasite, the dod- 

 der, which twines exten- 

 sively over coarse herbs, 

 sends into its host short 

 branches, likewise called 

 haustoria (fig. 652), 

 whose vascular strands 

 come into the most inti- 

 mate contact with those 

 of the host. (See Part 

 III on parasitism.) 



Partial parasites. 

 When such complete 

 contact has been estab- 

 lished, it is difficult to determine what or how much material migrates 



from host to parasite. Colorless parasites, of course, must get all 



their food from the host. Certain green parasites 



undoubtedly could live by getting merely water and 



its dissolved salts, for they can make food for them- 

 selves. Hence they are known as partial parasites. 



But that they completely restrict themselves to such 



food materials and do not admit any real food is 



quite improbable, in view of the intimate union 



between the two. 

 Mutualism. The support of the parasite by the 



host may result in no considerable injury or even 



weakening. Indeed, many cases have been described 



in which the association suggested a partnership, 



whence the term mutualism. From another point 



of view the relation resembles that of master and 



slave, whence the term helotism (see Part III). 



The lichens (p. 78) furnish the classical example. 



FIG. 653. Ecto- 



trophic mycorhiza of 

 European beech (Fa- 

 gus sylvatica) ; h t hy- 

 phae. After FRANK. 



Yet even here the algae are somewhat restricted in development by 

 the constant drain upon them, though perhaps they can work at food 



