408 PARASITES AND CARNIVOROUS PLANTS 



* 



379. Half -parasitic seed plants. Half parasites, or partial 

 parasites, are those which take a portion of their food, or of 

 raw materials to make food, from their host and manufacture 

 the rest for themselves. Usually they take mainly the newly 

 absorbed soil water from the host and do their own starch 

 making by combining the carbon dioxide, which they absorb 

 through their leaves, with the water stolen by the parasitic 

 roots, or li austoria, imbedded in the wood of the host. Evidently 

 the needed water may just as well be taken from the under- 

 ground parts of the host as from the upper portions, and accord- 

 ingly many half parasites are parasitic on roots. This is the 

 case with many of the beautiful false foxgloves (Gerardia), 

 with the painted cup (Castillea), and some species of false 

 toadflax (Comandra) and some orchids. 1 Usually these root 

 parasites are not recognized by non-botanical people as para- 

 sites at all, but in Germany a species common in grain fields 2 

 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. 

 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 chlorophyll 

 and so are not at all green. 



380. Wholly parasitic seed plants. These are so nearly 

 destitute of the power of photosynthesis that they must rob 

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

 the cancer root (Aphyllori) are root parasites ; others, like the 

 dodder (Fig. 16), 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 



1 See Bergen, Flora of the Northeastern States. 



2 Alectorolophits hirsutus. 



