SAPROPHYTISM AND SYMBIOSIS 795 



experiments on beech seedlings demonstrated that while the plants 

 developed vigorously in ordinary soil, ten out of fifteen seedlings grown 

 in sterilized soil died within two years, in spite of a greater than usual . 

 development of root hairs. Experiments with pine seedlings resulted 

 similarly, although no difference appeared between the cultures the first 

 year. 1 In contrast to these results it was claimed that species without 

 root fungi grow more luxuriantly in sterilized than in ordinary soil; 

 it should be said, however, that some investigators regard this last experi- 

 ment as inconclusive. 



Recently it has been demonstrated that orchids are dependent upon 

 their symbiotic fungi to a surprising degree. As a group, orchids have 

 been regarded as difficult of cultivation, and for a long time their seeds 

 were supposed to be incapable of germination. However, it has been 

 discovered that the thing requisite for germination is contact with the 

 appropriate fungus, in which respect orchids are comparable to such 

 holoparasites as Orobanche. Of great interest from the evolutionary 

 standpoint is the fact that various species of orchids differ in the degree 

 of their dependence upon their fungi. For example, the seeds of Bletia 

 germinate without fungus contact, and the seedlings continue to grow 

 as autophytes for several months, when growth ceases, never to be re- 

 sumed, unless the appropriate fungus comes in contact with the orchid. 

 In Laelia and Cattleya the autophytic seedling period is much shorter, 

 while in Cypripedium and Neottia, fungus contact is necessary for the 

 initial phase of germination. In nature the fungus almost always enters 

 the young seedling at the outset. When fungi are introduced into cul- 

 tures of minute orchid seedlings, a synthesis of hitherto separate indi- 

 viduals takes place that is altogether comparable to the formation of 

 lichens through the synthesis of algae and fungi (p. 800) . 2 



In the orchid mycorhizas there is a marked parallelism between the development 

 of the fungi and that of the orchids, the more generalized fungi being associated with 

 those orchids in which symbiosis is intermittent and most nearly facultative. The 

 higher orchids, on the other hand, are the most obligate of mycophytes, and their 

 fungal symbionts are relatively specialized forms. Apparently the development of 

 the two symbionts has been parallel, each becoming more intimately associated 

 with the other, as its evolution has progressed. The more generalized fungi, such 

 as Rhizoctonia repens, can infect a number of the lower orchids, while the more 



1 Pines may thrive even for several years without mycorhiza when grown in sterilized 

 humus. 



2 A similar synthesis has been effected by inoculating the roots of beech seedlings with 

 fungi taken from other beech roots. 



