8 THE FLOWERING PLANT. 
functions. And again, different members may be similar organs, 
and hence display analogy. Thus, among climbing plants, the 
ivy climbs by its roots, the convolvulus by its stem, the pea by 
modified leaves, and the hop by both stem and hair-structures. 
Structure of Plants.—Careful examination of plant-members 
shows that they are not homogeneous in structure. A thin skin 
can be peeled from the upper and lower surfaces of most leaves, 
and then comes a green pulp, traversed by firmer strands, popu- 
larly called nerves or veins. They are the parts which make 
up ‘‘skeleton ” leaves, the softer structures having rotted away. 
These different components of leaves, &c., are called tisswes, and 
it can be shown by the microscope that they, in their turn, are 
made up of smaller parts known as cells and cell-derivates. 'These 
cells, which are the units of plant-structure (just as bricks and 
stones are in a building), are for the most part microscopic, 
though sometimes of large size. The pulp of an orange, for 
example, is made up of such large cells, resembling in this case 
spindle-shaped bags, and containing a good deal of fluid. Botani- 
cal knowledge is based on the structure and physiological powers 
of cells, and the very simplest plants known (and simplest animals 
too) are single cells, in other words, they are unicellular. 
Life under Simple Conditions. — Without reference to any 
special example, we will consider the structure, and conditions 
of life, in one of these simplest forms, and then see what modifi- 
cations exist in multicellular plants, 7.e., those made up of many 
cells. | 
The general form of the body in our ideal example is spherical, 
and this appears to be a common result of uniformly distributed 
external influences. The shape is distinct and permanent, owing 
to the presence of a firm elastic membrane, the cell-wall, which 
forms a kind of superficial skin. Within the wall, constituting a 
lining to it, comes a slimy layer, the protoplasm, and part of this 
is of different texture from the rest, forming an obvious round 
or oval nucleus, which again contains a nucleolus. There are also 
present a number of grains of spongy protoplasm, permeated 
with chlorophyll, a green colouring matter. ‘These are chlorophyll 
granules. The larger internal space or vacuole is full of fluid, the 
cell-sap, which consists of water holding various substances in 
solution. It is also supposed to permeate the protoplasm and cell- 
wall. <A cell from the pulp of an orange corresponds fairly well 
with this description, but a slight modification of shape has taken 
place. So much for the morphology. The most essential phy- 
siological fact with which to start is that the protoplasm is the 
only living part of the cell. Very young cells contain no vacuole, 
which is produced later on, owing to the fact that growth in 
