THE LIFE OF A PRIMROSE. 1^1 
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 sun- 
light 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- 
phyll," will have nothing to do with the green waves 
and throws them back, so that every little grain of 
this protoplasm looks green . and gives the leaf its 
green colour. 
It is these little green cells that by the help of the 
sun-waves digest the food of the plant and turn the 
water and gases into useful sap and juices. We saw 
in Lecture III that when we breathe-in air, we use 
