ORGANIC RESPONSE 293 



Higher plants in several instances have adopted 

 parasitic habits and have thereby become profoundly 

 modified, usually in a " degenerative " way. The 

 dodder is a climbing plant related to the morning 

 glory. Its seeds germinate in the usual way, but 

 soon the seedling attaches itself to another plant, 

 and casting off its attachment to the ground, feeds 

 thereafter on the juices of its host, which it sucks 

 up through organs called haustoria, that grow fast 

 to the leaves and stems of the host-plant. The 

 mistletoe is another (semi-) parasitic seed plant that 

 grows upon the oak. Unlike the dodder it has no 

 ground-roots, the seeds being carried by birds 

 from tree to tree, where they catch in crevices in the 

 bark, and sprout. 



Association of Plants and Animals. Nearly all 

 the grades of association that have been described 

 'for plants or for animals are also found to exist 

 between plants and animals. 



Many crabs are protected by growths of seaweeds 

 which cover their carapace. When these are re- 

 moved, the crabs plant others. Although the algae 

 are passive members of this partnership, yet, of 

 course, they do not suffer by the association. 



In other cases, certain algae exist in a combination 

 with simple forms of animal life that is quite analo- 

 gous to the fungus-alga relation just described for 

 the lichens. The yellow variety of Hydra owes its 

 color to the presence within its endodermal cells 

 of a symbiotic alga. Ciliate Protozoa, sponges, and 



