3 82 
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 hdotism (see Part III). 
The lichens (p. 78) furnish the classical example. 
Yet even here the algae are somewhat restricted in development by 
the constant drain upon them, though perhaps they can work at food 
FIG. 653. Ecto- 
trophic mycorhiza of 
European beech (Fa- 
gus sylvatica) ; h, hy- 
phae. After FRANK. 
