Rayner.—Obligate Symbiosis, in Calluna vulgaris . 111 
impossible that such differences may be related with inconsistencies with 
regard to infection, which, as mentioned above, are sometimes observed in 
germinating seeds. Further, since fungal infection determines the develop¬ 
ment of the seedlings, it may also be correlated with observed irregularities 
in the distribution of the plant, and with the rate at which heather can become 
re-established on a given area by means of seed. 
The Origin of Ovarial Infection. 
The question at once arises—whence come the hyphae which infect the 
interior of an unopened fruit ? 
Two possibilities evidently exist: 
1. Local infection of the ovary or young fruit, from air-borne spores 
which reach the stigma independently, or with the pollen. 
2. Infection from the root upwards, involving a distribution of the 
mycorrhizal Fungus throughout the plant. 
The latter hypothesis has proved to be the correct one, and the facts 
now adduced in support of it involve the existence of a delicately balanced 
symbiosis of a kind not hitherto observed in any mycorrhizal plant. 
The facts to be described may be summarized in advance : 
1. When infection by the Fungus takes place at, or soon after germina¬ 
tion, all parts of the seedling—root, hypocotyl, and cotyledons—are invaded 
by fungal hyphae. 
2. In older seedlings, and in the mature plant, mycelium of the same 
Fungus occurs in all parts of the sub-aerial organs, in intimate relation with 
the tissues of the plant, and of the same nature throughout. Infection of the 
leaves is characteristic, and suggests a very delicate adjustment of the relations 
between leaf-cells and Fungus. 
Infection of the ovary and other parts of the flower is, therefore, only 
a special case of a condition common to all the vegetative parts. 
The observations on which these facts are based will be stated as briefly 
as is consistent with clearness, rather than in the form of a detailed cytological 
study. 
Methods. 
Fixation. A number of fixatives were tried with varying success, 
e. g. absolute-acetic, picric-formol, and cliromo-acetic solutions of various 
strengths. The best results were obtained by the use of Carnoy’s fluid, 
which has been used for the greater part of the work. 
The use. of a mixture of lactic acid and phenol as a clearing and 
mounting agent for whole roots, &c., has been found of great service, and is 
invaluable for the recognition of mycelium in fresh or unstained tissues. 
