FLORIDA STATE HORTICULTURAL SOCIETY 
35 
ditions, it is necessary that we know 
something about the tree itself in order 
that we may know how they influence 
it. The tree as a living organism has 
a definite structure. This structure 
may in a way be compared to that of 
a brick wall. The unit of structure in 
the wall is the brick. The unit of 
structure in the tree is the plant cell. 
As the collection of bricks makes the 
wall, so the collection of cells makes 
the tree. The difference between the 
brick and the plant cell is that the brick 
is large and contains only dead mineral 
matter, whereas the plant cell is so 
small that it cannot be seen by the un¬ 
aided eye and contains living matter. 
The plant cell itself has a definite 
structure and the arrangement of its 
parts may be compared to those of an 
egg. The shell of the egg represents 
the wall of the plant cell; the white 
of the egg, the living matter in the 
plant cell which is known as proto¬ 
plasm ; and the yellow of the egg, an 
open space or open spaces within the 
living mass known as vacuoles and 
which are filled with a watery solution 
of chemical compounds. 
All living plant cells have about this 
arrangement of their parts, but cells in 
different parts of the plant differ in 
their shape. For example, some of the 
cells that make up the wood are long, 
their walls are thick and in many cases 
the protoplasm has disappeared and 
the cells are dead. The cells of the 
bark vary in shape from those that are 
large round and have thin walls to 
those that are long and have more or 
less thickened walls; the protoplasm 
for the most part consists of a thin 
layer within the wall. The cells in the 
root tip are not so large, but are more 
or less rounded and are almost com¬ 
pletely filled with the protoplasm. 
Back of the tip, the cells do not contain 
so much protoplasm and the spaces 
within the protoplasm known as vacu¬ 
oles are larger. 
The cells vary thus because of the 
different work they have to do. Those 
back of the root tip have to do with the 
absorption of the solutions from, the 
soil that contain the raw food materi¬ 
als; the cells of the wood carry the 
water with the food materials in it up¬ 
ward for distribution to different parts 
of the tree; the cells of the bark carry 
the prepared plant foods downward for 
distribution to the different parts where 
it is used or stored. 
For the maintenance of life, doing 
its work and reproducing itself, the 
tree like any other organism, has cer¬ 
tain physiologic functions which it must 
perform. It breathes or respires. Oxy¬ 
gen is absorbed through the breathing 
pores, chemical compounds are broken 
down, energy is liberated and carbon di¬ 
oxide is freed. 
It transpires or evaporates water from 
its surface. In this process, water is ab¬ 
sorbed through the roots, and evaporated 
from the breathing pores which are lo¬ 
cated at various places over the surface 
of the plant body, particularly the leaves. 
Large quantities of water are thus ab¬ 
sorbed and given off, and in the move¬ 
ment of water through the tree, raw food 
materials are carried from the soil and 
distributed to the proper places in the 
