THE LIPE OF THE CELL. 113 
highest plants of all still show these two portions, but with 
far more differentiation. ‘The root and the shoot each con- 
sists of its various members; a root has lateral branches, a 
shoot, stem and leaves, and each member is composed of 
thousands of cells arranged in groups known as tissues; thus 
in the Flowering Plant there is the wood, a tissue not existing 
in such a lowly organised plant as the Moss. ‘This process 
of differentiation, by which groups of cells become set apart 
for a particular work, is the feature that marks off a higher 
from a lower plant ; the higher the plant the more marked is 
its differentiation. This is most strikingly seen in reproduc- 
tion. In Protococcus, the whole unicellular plant divides, 
giving rise to two (or more) organisms. In the Flowering 
Plant, a ee shoot is modified for the production of the 
seed. 
The important point to emphasise here is, that every plant, 
however highly organised, and every part 
(member) of a plant consists of cells. 
Fig, 136 shows a typical vegetable cell, 
such as may be obtained from the pith of 
_ Elder in early spring. It consists of pro- 
toplasm with nucleus and nucleolus, of a 
cell-wall, and as it is a young cell the 
protoplasm, which is saturated with cell- 
sap, fills, or almost fills, the whole cell. 
As the cell grows older the cell-wall Fy¢.136.—A rvprca 
grows more rapidly than the protoplasm ee ec 
can be built up, so that the protoplasm  cell-wall; v, vacuole ; 
comes to form merely a lining to the ™ nucleus, showing 
nucleolus. 
cell-wall. The space thus formed within 
the cell is the vacuole, and is completely filled with cell-sap. 
In the following chapters, the processes that go on in 
the plant as a whole are described as far as possible experi- 
mentally, and without reference to the minute structures of 
the tissues which cannot be satisfactorily studied without 
the constant use of a microscope ; but it has been necessary 
in this introductory chapter to Plant Physiology to try and 

