PARASITES 337 



all, but in Germany a species common in grain fields ^ and 

 the eyebright, which abounds in grass fields, are respectively 

 known as "hunger" and "milk-thief," from the injury 

 they do to the plants on which they fasten themselves. 

 The mistletoe is a familiar example of a half-parasite, 

 which roots on branches (Plate IX). Among the scanty 

 belts of Cottonwood trees along streams in New Mexico it 

 is necessary to lop off the mistletoe every year to give the 

 tree any chance to grow. Half-parasites may be known 

 from plants that are fully parasitic by having green or 

 greenish foliage, while complete parasites have no chloro- 

 phyll and so are not at all green. 



406. Wholly Parasitic Seed-Plants. — These are so nearly 

 destitute of the power of assimilation that they must rob 

 other plants of all needed food or die of starvation. Some, 

 like the cancer-root (see Flora), are root-parasites ; others, 

 like the dodder, are parasitic on stems above ground. The 

 most dependent species of all, such as the flax-dodder, can 

 live on ,only one kind of host, while the coarse orange- 

 stemmed dodder,^ which is common all over the central 

 and the northeastern states, grows freely on many kinds 

 of plants, from golden-rods to willows. 



407. Parasitic Cryptogams. — The wheat rust (Sect. 810) 

 affords an excellent example of the relations between 

 parasitic fungi and their hosts. The illustration showing 

 the potato blight escaping from a stoma of the potato leaf 

 (Fig. 191) shows plainly one way in which a microscopic 

 parasite finds its way out of the tissues of the host-plant 

 to ripen and scatter its spores. 



1 Alectorolophus hirsutus. 

 s Oiiscuta Qronovii, 



