io6 
Rayner.—Obligate Symbiosis in Calluna vulgaris . 
to be drawn 
through 
the sand in the tubes 
cultures, which allowed air 
periodically (Text-fig. i). 
These tubes proved satisfactory and remained sterile for several 
months, but gave no better results than the non-aerated sand cultures. In 
no case was a root-system developed. Sterile 
seedlings from these sand cultures resembled 
those from agar cultures (Text-fig. 2). 
Unsterilized controls cultivated for several 
months in these tubes grew slowly but de¬ 
veloped a normal root-system (PI. VI, 
Fig. 10). 
For agar cultures, sterile seedlings were 
transferred to tubes containing a solution pre¬ 
viously used with success for water cultures, 2 
made up with 012 per cent. agar. These 
cultures have an advantage over those grown 
in sand in that infection of the medium by 
micro-organisms is at once evident. Such 
tubes have been maintained in a sterile con¬ 
dition for ten months. 
The seedlings usually formed a few leaves, 
chlorotic or reddish in colour, but did not 
develop roots, although they remained in a 
turgid condition and apparently alive for five 
to six months (Text-fig. 2). 
The cultivation of unsterilized controls in 
agar presents some difficulty owing to the 
vigorous development of the epiphytic micro¬ 
flora associated with the roots. Otherwise the 
growth of such seedlings is normal, and com¬ 
parable with that of those grown in soil, 
e. g. the shoot-system of such a seedling had 
reached a height of 5 cm. four months after 
planting. 
Text-fig. i. Apparatus for 
growing sterile sand cultures. 
/1 -f 3 = Massen filter candles; 
^i-^ 6 = rubber corks ; a = glass 
cap for attachment to aspirator; 
b = glass cap to exclude dust; 
k = sand ; p — cotton-wool plug ; 
s = nutrient solution ; x — wax 
joint. 
1 The water-culture solution known for the sake of brevity as Solution A was one in which 
unsterilized seedlings grow vigorously ( 3 , p. 60). It formed the basis of all media used in experi¬ 
mental cultures. The composition is as follows : 
Potassium nitrate (KN 0 3 ) . ..i*o grm. 
Magnesium sulphate (MgSO t ) 
Calcium sulphate (CaSO±) . 
Calcium monophosphate (CaH 4 P 2 0 8 ) 
Sodium chloride (NaCl) 
Ferric chloride .... 
Water ...... 
(Acidity: 0*012 normal.) 
0*4 grm. 
0*5 grm. 
0*5 grm. 
0*5 grm. 
Trace. 
2,000 c.c. 
