THE LIFE OF A PRIMROSE. 157 
as treacle and water for example, are only separated 
by a skin or any porous substance, they will always 
mix, the thinner one oozing through the skin into 
the thicker one. If you tie a piece of bladder over a 
glass tube, half fill the tube with treacle, and then let 
the covered end rest in a bottle of water, in a few 
hours the water will get in to the treacle and the 
mixture will rise up in the tube till it flows over the 
top. Now, the saps and juices of plants are thicker 
than water, so, directly the water enters the cells at 
the root it oozes up into the cells above, and mixes 
with the sap. Then the matter in those cells becomes 
thinner than in the cells above, so it too oozes up, 
and in this way cell by cell the water is pumped up 
into the leaves. 
When it gets there it finds our old friends the sun- 
beams hard at work. If you have ever tried to grow 
a plant in a cellar, you will know that in the dark its 
leaves remain white and sickly. It is only in the 
sunlight that a beautiful delicate green tint is given 
to them, and you will remember from Lecture II. that 
this green tint shows that the leaf has used all the 
sun-waves except those which make you see green ; 
but why should it do this only when it has grown up 
in the sunshine ? 
The reason is this : when the sunbeam darts into 
the leaf and sets all its particles quivering, it divides 
the protoplasm into two kinds, collected into different 
cells. One of these remains white, but the other kind, 
near the surface, is altered by the sunlight and by the 
help of the iron brought in by the water. This par- 
ticular kind of protoplasm, which is called "chloro- 
