TO THE READER 
HE sign X with a number after it indicates the 
magnification : thus, for Fig. 1, Plate I., p. 10, 
x70 means that the photograph is seventy times 
as long and seventy times as broad as the piece 
of leaf-skin itself. It would therefore take four 
thousand nine hundred such pieces, or seventy rows 
of seventy each, to cover the photograph without 
leaving any spaces between them. 
If, however, we turn to the ilustration of the 
seed of the Ragged Robin Fig. 26, Plate II., p. '70, and 
arrange thirty rows of thirty seeds apiece, we shall more 
than cover the photograph. We shall have a figure 
which is not the same shape as the seed, but a rectangle 
which will overlap the photograph at the angles, 
because there will be spaces between the seeds, and 
the total area of these spaces will exactly equal the 
total area of the overlap. In other words, if we draw 
four straight lines at right angles to one another 
around the photographed seed so as just to touch it 
on each side, we shall find that the nine hundred 
seeds will just cover the rectangle. 
The reader will be well advised to look at the 
illustrations, and especially the photographs, through 
a low-power reading glass. 
xi 
