122 Rayner.—Obligate Symbiosis in Calluna vulgaris . 
2. Sterile seedlings, inoculated with this Fungus (from either source), 
subsequently produce normal root and shoot systems when grown in closed 
tubes, under aseptic conditions. 
3. Control seedlings remain rootless. 
4. The ease with which plant and Fungus can be successfully synthesized 
depends upon the nutritive and other conditions under which the experiment 
is conducted, the age of the seedling, the age and vigour of the fungal 
culture used, and, in all probability, upon the composition of the nutrient 
material upon which the latter was cultivated outside the plant. 
5. The Fungus isolated in this way is morphologically identical with 
that present in the root mycorrhiza. 
The Endophyte. 
Inside the plant. The vegetative mycelium as it appears in the 
various organs of the plant has already been fully described (p. 112). With 
the exception of the characteristic clusters of branches which fill the root- 
cells (PI. VI, Fig. 11), and the swellings on hyphae outside the root but 
continuous with those in the tissues (PI. VI, Fig. 3), no special organs 
have been observed. There is no doubt but that the distribution of the 
endophyte in the artificially synthesized plant is as wide as that described 
for normal seedlings ; there is no reason to believe that the details of 
development in the tissues differ in any respect from those described for 
the latter. 
The formation of tubercles. Invasion of the roots of a vascular plant 
by either Fungi or Bacteria is often marked by the formation of tubercles 
or nodules. The root tubercles of Leguminosae and the nodules on the 
roots of Alnus glutinosa and Podocarpus are familiar examples. 
So far as I am aware, there is no record of tubercles on the roots 
of an ericaceous plant, and their formation must, therefore, be relatively 
rare. 
Tubercles of a quite characteristic kind are, however, formed sometimes 
by healthy plants of Calluna growing on typical heath soils. 
The tissues of these tubercles contain fungal hyphae and Bacteria. The 
details of their structure and the relations, if any, between the two micro¬ 
organisms is being investigated and may throw light on the nature of the 
bacterial colonies present on the roots when growing in certain soils. 
Outside the plant . Sub-cultures of the Fungus from the original colonies 
derived from the plant tissues were made on rice and on gelatine- Calluna 
extract. 
The behaviour of the organism on these and other media, together 
with a detailed account of the morphological characters, will be found in 
the Appendix (p. 128). 
