68 THE OAK 
the above cells are found to be septate and cut up into 
parenchyma-like cells—irregular bast-parenchyma. The 
walls, especially the longitudinal walls, are marked 
either with crowded small pits, giving a reticulate ap- 
pearance, or have sieve-plates; all intermediate stages 
occur also. The transverse walls are also pitted with 
sieve-plates. 
All the cells of the soft bast contain tannin, and 
small grains which turn brown in iodine (leucoplasts ?). 
Very little starch is found in them except in winter. 
Crystals occur in pitted cells here and there (fig. 18, 
d and e). 
Kyven in the first year the cambium may produce 
small groups of thick-walled bast fibres of exactly the 
same character as those of the primordial groups. 
It is obvious that while the wood elements remain 
fixed in the cylindrical surface where they are developed, 
the bast elements formed outside the cambium, being 
driven outwards in consequence of growth in thickness, 
come to lie ina layer of continually increasing radius. If 
these bast elements were unyielding and lignified there 
would be a solid sheath of elements which refused to 
extend by mechanical distension, cell division, or growth 
of cell-walls; this would finally rupture under the 
pressure from within. This is prevented by the division 
and growth of the chief phloém elements. 
In the vascular-bundle system of the stem there are 
no essential differences in structure as we pass from one 
region to another; the only variations are in the thick- 
