Rayner.—Obligate Symbiosis in Cal lima vulgaris. 115 
A summary of the chief features of general anatomical interest in 
various genera has been given by Solereder ( 37 ). 
The details of leaf structure in the members of the sub-order Ericoideae 
have been the subject of a monograph by Ljungstrom ( 38 ), who used his 
observations as a basis for subdivision of the group into four classes, each 
distinguished by characteristic leaf structure ; in the fourth of these classes 
he places Calluna together with Erica dianthifolia. 
The Vaccinioideae, Arbutoideae, and Rhododendroideae have also been 
monographed from this point of view by other observers ( 39 , 40 ). 
Contributions to the comparative anatomy of the members of the order 
have been made also by Simon ( 41 ). 
These observers have drawn attention to various features of interest in 
the anatomy of the leaf: the characteristic shape, with corresponding 
anomalies in the distribution of the assimilating tissues; the complicated 
structure of the cell-walls and cuticle; the distribution and structure of 
many different types of covering and secretory hairs; and the presence 
of accumulations of calcium oxalate in the tissues of the stem and leaf, 
either as single crystals or crystal-aggregates. 
There is no record in these papers of any observations dealing with 
a regular infection of the tissues of the shoot by fungal hyphae, such 
as is about to be described. 
In view of the very full accounts given in the papers cited above, 
a brief summary of the anatomy of the leaf will suffice. 
The various species of Erica and its allies have often been described as 
possessing 4 rolled leaves ’, a misleading title since the characteristic hollow¬ 
ing of the under side does not arise by a rolling back of the upper side of the 
leaf, but is developed secondarily as a groove or furrow (or several such) on 
the lower side. 
The leaves of Calluna are decussate in arrangement, of small size, and 
of the familiar ericoid type. The mature leaf is almost quadrangular in 
section, the sides tapering gradually to the abaxial surface, of which most 
of the width is occupied by a single conspicuous groove, the walls of which 
are beset with hairs. 
The adaxial side of the leaf is adpressed to the stem, and owing to this, 
and to the shape of the leaf, the flanks receive most illumination ; correlated 
with which the bulk of the chlorophyllous tissue is found beneath the 
epidermis in this region. 
Stomata are present on both sides of the leaf. 
The assimilating tissue in the mature leaf has lost all trace of the dorsi- 
ventrality found in the cotyledon, and the arrangement of the assimilating 
tissue—as in other ericoid types—is closely correlated with the shape and 
position of the leaf, and its consequent illumination. 
Chlorophyllous tissue is practically absent from the upper side of the 
X % 
