18 TYPES OF BRITISH PLANTS 
tight-packed layer of cells, but as soon as spring comes 
the layer is split and the air can come through again. 
A very good example of this cork jacket is seen in the 
rind of a potato, which is not really a root, but a thickened 
underground stem. If you cut off a slice of the rind, 
leaving the white surface exposed, the living cells under- 
neath will set to work and make brown cork-cells which 
will soon cover the whole of the exposed surface. 
I am now free to deal with the question of the vessels 
and the bundles, and, as this is perhaps the most difficult 
point to make clear, I must for a moment have your 
special attention. 
In the first place, what is a vessel? Remember this : 
it is not just an overgrown or elongated cell, but it is a 
combination of a whole long row of cells. Imagine a 
row of the boxes to which cells were compared placed 
side by side in a straight line, and then the sides of the 
boxes slowly disappearing, or being absorbed, until the 
boxes had become one long tube, running, in many cases, 
the whole length of a plant. Sometimes, instead of the 
partitions disappearing altogether, they are merely per- 
forated with many holes like a sieve. It is not worth 
while here to go into ‘all the different kinds of vessels, 
the use of some of which we do not know. Some 
conduct air, some water, and some sap about the plant, 
far more quickly and effectively than the cells could do 
it, when the material wanted, air, or water, or what not, 
would have to percolate through the cell-wall. In some 
plants, such as the Dandelion, there are also vessels which 
branch in all directions, and are full of that milky juice 
which you will see oozing out from a broken Dandelion 
stalk. This white juice is very often useful to mankind, 
as you will believe when you know that indiarubber is 
