THE LEAF 117 
these is a layer of green spongy material, the mesophyll 
(Gk. mesos in the middle and phyllon a leaf), which 
is traversed by the veins. Another good way 
of separating the tissues is to boil for ten or fifteen 
minutes in caustic soda some small firm leaves that 
have un-notched edges, as, for instance, those of the 
box and privet. Holding one of these under water 
cut off a strip round the edge and then, with a pair 
of needles mounted on wooden handles, separate it 
into upper and lower epidermis and mesophyll. 
To examine further into leaf structure demands 
the use of the microscope and an ability to eut sections 
such as could not reasonably be expected of the 
students who will use this book. So important, 
however, is this matter, and so closely related is 
structure to funetion, that detailed drawings are 
supplied. For those who have the time and patience 
to acquire the necessary manual dexterity, and possess 
a microscope to examine its result, the following 
instructions will prove useful. Roll a bean leaflet into 
a firm coil, and, holding it lightly so as not to erush 
the tissue, eut the top off the coil in order to obtain 
a flat surface. Now, using a very sharp razor, and 
being eareful to keep both razor and coil quite wet, 
slide the blade over the flat surface without trying to 
eut anything. Sections may in this way be obtained, 
as it were by accident. With a ecamel’s hair brush 
remove these to a glass slide, mount in water, and 
examine under both low and high powers. In 
preparing for examination the epidermis that has been 
peeled off no such difficulty is encountered. It is merely 
mounted in water, flat on the slide. 
In the leaf sections (Fig. 80) it will be noted that 
the outside walls of the cells of the upper epidermis 
are thicker than in those of the lower, as might be 
expected from the necessity for greater proteetion in 
the more exposed position. In the lower surface, and 
