io8 Rayner.—Obligate Symbiosis in Calluna vulgaris. 
which retain their green colour for several months ; e. g. a large proportion 
of them were alive and green at the end of five months. The starvation 
symptoms—arrest of roots, yellowing of leaves, &c., characteristic of sterile 
seedlings shortly after germination—are absent (Text-fig. 3). 
It was thought possible, therefore, that the failure to form roots 
by uninfected seedlings might be correlated with some disturbance of 
nitrogen metabolism in the plant, assuming interchange of nitrogenous 
material from Fungus to plant to be a feature of the symbiosis under normal 
conditions. 
Although without direct evidence to support this hypothesis, it seemed 
desirable to ascertain whether a supply of organic nitrogen would induce 
root formation, or postpone the appearance of starvation symptoms in 
uninfected seedlings. To test this possibility, two parallel series of sterile 
sand and agar cultures were carried on during several months. 
The solutions used for watering the sand cultures were as follows: 
Series a. Solution A; o-i per cent, dextrose; o-i per cent. Witte’s 
peptone. 
Series ( 3 . Solution A ; o-i per cent, saccharose; 0-032 per cent, 
asparagin. 
Series y. Solution A ; o-i per cent, dextrose ; 0-04 per cent, uric acid. 
Series b. Solution A ; o-i per cent, saccharose ; 0-03 per cent, 
glycocol. 
In a parallel series of agar cultures, solutions of 0-3 per cent, total 
concentration of Solution A were used in each case, made up with the 
addition of 0-12 per cent, powdered agar. 
Since the result of these cultures was in every case negative, the 
experiments are not recorded in detail. In each series, tubes remained 
sterile for several months—as judged by absence of cloudiness in the 
media—e. g. agar tubes planted August 13, 1913, were still sterile on 
April 30, 1914. 
The vitality of the seedlings on the different substrata varied. The 
only solution which appeared to be actively injurious was that used in 
Series b to which glycocol had been added, the seedlings planted in 
which showed rapid discoloration of the leaves, and succumbed after 
a few weeks. 
The greatest vitality was shown in the agar cultures of Series a and 
Series y, more especially in the latter. In both cases the tubes were 
sterile and the seedlings alive—although moribund—at the end of six 
months. 
The seedlings supplied with uric acid (Series y) showed signs of activity 
after planting, but the growth initiated in the early stages was not maintained; 
the hypocotyl elongated, several leaves were formed, and in one case the 
plant produced a few small abortive rootlets. The medium in this tube 
