556 Holmes.—A Study in the Anatomy of 
terminal bud was present; this is shown in Fig. 2. By reference to Fig. 1 
it will be seen that, of these four specimens, H8 showed the smallest number 
of internodes for the greatest length. 
Anatomy. The most apparent difference between the stool shoots and 
tree shoots as regards the internal arrangement of tissues consists in the 
much larger size of the pith in the former. It maintains a large size all 
through the middle part of the shoot, decreasing slightly towards the apex, 
and more rapidly towards the base, where it becomes quite small. The 
decrease in the width of the layer outside the cambium, consisting of phloem, 
cortex, and cork, is but slight, so that in the large transverse sections of the 
shoot, near the base, there is a disproportionately large amount of wood, 
compared with the smaller sections higher up the same shoot. The tissues 
in stool shoots and tree shoots in their first year are present in fairly similar 
proportions, but on the whole there is a greater proportion of pith and of 
wood in the former. The cylinder of wood decreases in width from base to 
apex ; it is hard and firm in the lower part of the shoot and becomes softer 
in the upper part. In the terminal internode the cylinder is not completely 
closed, being still interrupted by wide medullary rays. In sections from 
the terminal internodes of similar stool shoots cut in February the wood is 
more mature and the cylinder has become closed. 
The wood consists of dead lignified elements interspersed with living 
cells and divided radially into narrow sectors by medullary rays also com¬ 
posed of living cells. Near the apex of the stem the dead elements are 
almost all of a kind capable of conducting water, but lower down a large 
number of them are fibrous, while towards the base the fibres predominate 
greatly, the water-conducting elements being scattered sparsely among 
them. The elements formed first, i. e. next to the pith, are vessels with 
spiral thickening, and constitute the protoxylem ; these can be distinguished 
at all levels in the stem. In the upper part of the stem they carry a con¬ 
siderable proportion of the water, but lower down they become crushed, and 
their functions are assumed by the pitted vessels which are formed later. 
The vessels of the primary xylem have spiral thickening also; they are 
arranged in radial rows, and increase in diameter from the protoxylem 
outwards. At all levels in the shoot this region of the wood, immediately 
outside the pith, is richest in vessels. They are crowded together, with only 
a few fibres between them here and there. This represents the response of 
the wood to the first demand of the growing stem for water, and it is not till 
later that the wood begins to exercise its supporting function, and mechanical 
elements become differentiated in larger proportion. The widest vessels 
occur in the inner part of the secondary xylem. In the lower part of the 
stem the narrower vessels towards the periphery are associated with tracheides. 
Here the water-conducting elements are distributed in obliquely radial 
groups, the space between being occupied by mechanical and storage tissue 
