TO BIRD’s-EYE FORMATION IN THE WOOD OF TREES. 25 
a longitudinal section from the surface of the bottom specimen of 
Plate IX., Fig. 20. It consists of three photomicrographs of 
different portions taken separately and subsequently pieced 
together. The inward prolongation of the papilla is distinctly 
seen passing through the cambium and wood. It will be observed 
that here again the cambium-cells show the same incurving as 
do those of Lawson’s cypress and Thuja gigantea. The apex of 
the papilla appears hollow and torn. This is due to the resistance 
offered to its outward growth by one of the bundles of hard 
sclerenchyma-fibres which occur in the cortex. To the left in the 
cortex lie a few dark discoloured patches of sclerenchyma which 
have been displaced by the outgrowth. They have, however, 
offered a certain resistance which is indicated by the depression 
formed opposite to them in the protruding cone. 
Plate IX., Fig. 24, represents an early stage in the development 
of a papilla, which, although continued for some distance back 
through the wood, is not yet visible on the outside. The 
formation of the papilla commences with the broadening out of 
a medullary ray round the end of which the wood-cambium 
spreads. Simultaneously with this a conical cap of parenchyma 
is formed over the apex, which projects into the cortex of the 
stem. This parenchyma-mass seems to arise from division of 
the parenchyma-cells of the phloem. It may be seen in the 
_ figure arching out into the cortex and compressing its cells, so 
that the gradual attenuation of the cortex in front of this 
developing cone is quite apparent. The line of demarcation 
between the two tissues is indicated by the dark line of crushed 
cells. In the middle of the papilla occurs a triangular area 
consisting of a disorganised mass surrounded by one or two 
layers of meristematic parenchyma. The central mass is 
composed of somewhat displaced and crushed-up remains of 
sclerenchyma-fibres. 
A further stage of development is shown in Plate IX., Fig. 23. 
The parenchyma-cone has now become much larger and has 
reached the outer cork-layer through which it is about to break. 
Right and left lie the sclerenchyma-fibres and other tissues of 
1Sorauer, Schutz der Obstbaume (1900), p. 58, finds that meristematic 
cells may arise in a similar way round hard sclerenchyma-bundles in the 
cortex of the apple-tree. 
