Rayner.—Obligate Symbiosis in Calluna vulgaris. 129 
bared parts of the surface, indicating the formation of pycnidia. Older mycelium 
greyish brown; younger mycelium from germination of the pycnidiospores snow-white. 
Mycelium resembling that found in the root mycorrhiza of the plant. Larger hyphae 
hyaline, regularly septate, yellow-brown, diameter 0*007-0-008 mm., giving rise directly 
or by gradual transition to fine, colourless, branched hyphae; the ultimate branches 
very attenuated. 
Larger hyphae rarely shortly jointed in growth or swelling to form terminal or 
intercalary swellings; often forming strands of parallel-growing hyphae. The colour¬ 
less vegetative hyphae become densely interwoven, but there is no formation of 
a definite stroma. 
Pycnidia produced freely on the surface; globose or slightly irregular in shape, 
with apical papilla; size variable, diameter from 0*2 mm. upwards; colour dark 
brown (PI. VI, Fig. 8). 
Pycnidiospores oval, 0*003 mm.-0*004 x 0*002-0*003 mm. ; wall pale greenish 
yellow to yellow-brown (PI. VI, Fig. 9). 
2. Calluna-Extract Gelatine} 
Ten days culture. Colony 4 cm. diameter. Superficial growth of mycelium, well 
developed and snow-white in colour. 
Fifteen days' culture (PI. VI, Fig. 20). Colony 6*5 cm. diameter. Mycelium 
snow-white. Gelatine slowly liquefying. 
Mycelium vigorous; hyphae branched, septate, colourless to yellow-brown, resem¬ 
bling those from culture on rice, but in older cultures the majority form characteristic 
swellings—intercalary, terminal, or in chains. Mycelium often forming parallel strands. 
No formation of spores or pycnidia observed. 
Complete liquefaction of the gelatine in older cultures. 
3. Water-culture solution A (p. 106) and 0*12 % Agar. 
Twelve weeks' culture. Medium slightly clouded, colour unchanged. Growth on 
surface feeble or absent, except for a slight growth of hyphae on the sides of the 
tube. Mycelium homogeneous, composed of fine, branched, colourless hyphae, 
frequently anastomosing. The mycelium in this culture was identical with that 
observed in the early stages of infection of sterile seedlings. 
4. Solution A + o* 1 % Dextrose +o*i % Witte's Peptone + 0*12 % Agar. 
Twenty days' culture. Colony 7 cm. diameter. 
Thirty „ ,, Colony covered plate. 
Growth colourless; scanty white mycelium on surface. 
In older cultures the medium darkens very slowly, becoming yellow to yellowish 
brown. 
Growth vigorous, chiefly below surface. Hyphae colourless to yellow-brown in 
old cultures. Older hyphae often closely septate and with thicker walls; no separa¬ 
tion of chlamydospores observed. No formation of swellings in cultures up to five 
months old. Refractive droplets abundant in hyphae and as an excretion in the 
medium. No formation of spores or pycnidia observed. 
1 1 30 grm. of young shoots of Calluna extracted in water ; extract made up to i litre, and 
120 grm. sheet gelatine added = dark-brown medium. 
K 
