20 TYPES OF BRITISH PLANTS 
we call the veins of a leaf are really the extreme ends 
of these bundles, which run up to the very tips, giving 
strength to meet the wind, the vessels also acting as 
conductors to and from the leaf-factories. Irom the 
spreading leaf they run down the stalk and into the 
stem. If you take a leaf of the ordinary plantain, 
which you may find, unfortunately, killing the grass on — 
nearly every tennis lawn or cricket pitch, and pull the 
leaf-stalk in two, you will find that there are pale 
threads, very tough, hanging from one of the broken 
pieces. These are the bundles hastening down to join 
the stem, and under a strong magnifying-glass you would 
be able in a cross-section to see the three elements, wood, 
ecambium, and bast. 
The wood section consists chiefly of small cells with 
hardened walls and a few large vessels, generally with 
deeply-pitted sides; the bast section contains some of 
the sieve tube vessels which we mentioned, and some 
tough fibres, which in trees form the bast which we use. 
Beyond the bast generally lies a row of cells which is 
called the bundle-sheath. These cells are usually a rich 
reservoir of starch grains. 
In between the wood and the bast sections comes the 
all-important cambiwm layer. From this layer both the 
other sides spring, for its cells are continually splitting, 
sending off on one side cells to form wood and its vessels, 
on the other cells to form bast and its vessels. 
You will remember that, when we were talking of 
classification, the number of leaves in a seed seemed 
hardly sufficient difference to mark off a class (p. 8), 
but that other and very important differences went with 
it. One of these differences arises over the question of 
these bundles. The Monocotyledons (plants with only 
