Rayner.—Obligate Symbiosis in Calluna vulgaris. ioi 
seem to be open to criticism in the light of recent researches, and is further 
discussed at the end of this paper (p. 125). 
In recent years the mycorrhiza of the Orchids has attracted attention 
owing to the abundance of the endophyte in the tissues, and to its very 
characteristic relations with the cells of the plant. 
The researches of Bernard mark an epoch in the knowledge of endo- 
trophic mycorrhiza and of its biological significance. His discoveries were 
not only of great theoretical interest but are of some potential value to 
Orchid growers. 
It had long been known to horticulturists that the seeds of Orchids— 
especially of certain genera, e. g. Odontoglossum and Vanda —are extremely 
difficult to germinate. 
Working with a number of Orchid species, Bernard found that it was 
impossible to germinate seeds removed under aseptic conditions from steri¬ 
lized capsules. Some degree of development usually took place, but except 
in rare cases—e. g. Bletilla hyacinthina —the embryo did not reach an 
advanced stage. In no case, in his cultures, did development of the 
seedling reach the stage of root formation, unless injection from the 
substratum took place. 
The next step in the investigation was the isolation of the mycorrhizal 
Fungus ; seed cultures were inoculated from a pure culture of the Fungus so 
obtained, and normal Orchid seedlings were raised successfully by this means. 
Various transition stages with regard to the degree of dependence 
of the plant upon the Fungus were observed by Bernard. 
In Bletilla hyacinthina , a relatively unspecialized type, uninfected seeds 
germinate and the seedlings form several leaves, i. e. the plantlet reaches 
a comparatively advanced stage, but is unable to develop roots without 
infection. In other Orchids, development of the embryo is arrested at 
a much earlier stage. 
According to Bernard, the degree of specificity between plant and Fungus 
is variable : thus, using species of Cattleya and Cypripedium , he found that the 
Fungus isolated from a species of the one genus could be used successfully for 
inoculation of the other, but in the case of seeds of species of Phalaenopsis 
and Odontoglossum , which under normal conditions are difficult to germi¬ 
nate, the Fungus was found to be specific to the plant. It was determined 
further that Orchid species differ in respect of their ability to resist invasion 
by the mycorrhizal Fungus from another species. Thus, in the case of 
a species of Phalaenopsis , infection by the Fungus of Cattleya sp. killed 
the seeds outright—the plant cells made no attempt to resist the attack; in 
another species, infection took place, followed by digestion of the Fungus by 
the cells .of the embryo and subsequent arrest of development. 
In the Orchids, therefore, the symbiotic relation between plant and 
micro-organism is an obligate one, and has resulted in a high degree of 
