Rayner.—Obligate Symbiosis in Calluna vulgaris . 109 
was free from bacterial or fungoid infection, and had not lost an appreciable 
amount of water. 
All these cultures were grown in a small cool greenhouse, under the 
same conditions as normal seedlings in soil, special precautions being taken 
to prevent excessive loss of water from the tubes. 
It may be inferred from these experiments that the stimulus to root 
development which follows entry of the Fungus into the tissues cannot 
be replaced by supplying the seedling with organic nitrogen in the forms 
mentioned. 
3. The source of infection. Infection of the seedling root, shortly after 
germination, has been described in an earlier paper ( 3 , p. 68). 
The necessary observations can readily be made on seeds germinating 
on blotting-paper in closed dishes. Infection 
takes place by the outgrowth of delicate 
hyaline hyphae from the cells of the testa, 
simultaneously with — or shortly after — 
emergence of the seedling root. 
In the majority of seed cultures ob¬ 
served, infection has occurred with great 
regularity at this stage, and has been fol¬ 
lowed by the rapid development of a number 
of fine, transparent roots, arising adventi¬ 
tiously from the base of the hypocotyl. 
Certain irregularities have been observed 
nevertheless, of which at present no ex¬ 
planation can be given. 
In a few cultures, for instance, sown 
in the early winter, soon after collection of 
the seed, infection for some reason seemed to be inhibited or delayed. 
These seedlings remained in a rootless condition in the seed dishes for 
weeks, and soon showed marked browning and discoloration of the base of 
the hypocotyl (Text-fig. 4) ; removed to another dish and placed in contact 
with a normally infected seedling, they at once began to develop roots. 
In the paper already cited ( 3 ) it was stated that infection of the seed- 
coat takes place while the seeds are still in the ovary, and is independent of 
the bacterial and fungal infection from the air, to which all seeds are liable 
after they have been shed. 
In order to determine the source of this infection, seeds were removed 
from unopened capsules for examination, and the unripe fruits containing 
seed investigated by means of sections cut from fixed material, embedded 
in paraffin or celloidin. 
Thick sections of material prepared in this way make clear the manner 
in which infection takes place. 
imm. 
Text- fig. 4. Seedlings from ger- 
minator; delayed infection accom¬ 
panied by inhibition of root formation 
five months after sowing; about four 
months after germination. Camera 
lucida drawings. 
