The Anatomy of Ginkgo biloba. 89 
gradually approach one another. At the extreme top of the collar 
they meet and the cambium of the main bundle becomes continuous 
all the way round (Text-fig. 16, 9 ). The number of “transfusion 
tracheids ” in the general parenchyma is very large at this level. 
A single concentric bundle is thus formed at the base of the ovule ; 
its central part consists of the metaxylem, mentioned above, 
and it is enclosed by the genuine xylem of the bundle. The latter 
shows a marked tendency to aggregate at two points on opposite 
sides of the bundle (Text-fig. 16, 9 ). The tracheids in one half of 
the bundle all appear to be running towards the region where the 
“ horns” have recently united, in the other half of the bundle they 
trend towards a point exactly opposite. This behaviour of the 
tracheids is a foreshadowing of the splitting of the concentric 
bundle which takes place a few sections higher up in the ovule. 
Seeing that the two “ horns,” which united to form the concentric 
bundle, faced towards the sterile ovule, it is plain that the separation 
of this bundle into two takes place at right angles to the plane of 
the ovules—the two resulting bundles lying in that plane (Text-fig. 
16, 10 ). They rapidly divaricate and become first horseshoe-shaped 
and then mesarch and concentric (Text-fig. 16, 11 ). Ultimately 
they die out after passing up the side of the megaspore cavity for 
some little distance. 
The essential feature to which attention is drawn consists in 
the development of anomalous vascular tissue, with inverted 
orientation, throughout the region of the collar. This tissue is 
strictly confined to the collar and takes no part in the vascular 
supply of the ovule. 
Besides the anomalous tissue the peculiar manner in which 
the main bundle becomes concentric is worthy of notice. Stress 
has already been laid on the fact that throughout this process its 
cambium remains continuous with that of the anomalous tissue ; 
i.e., while the “ horns” approach one another, during the passage of 
the bundle upwards, the cambium remains continuous round the 
“ horns.” Since these approach one another fairly rapidly, as is 
shewn by the fact that their tracheids run longitudinally in the 
transverse sections, it follows that a sort of oblique gutter of 
cambium is formed on either flank of the main bundle. The outer 
wall of the gutter is the cambium of the anomalous tissue, the inner 
wall that of the main bundle. A consideration of the description 
together with the photograph of the model will make this clear 
(Text-fig. 18). 
