A GLIMPSE OF THE LIFE OF CHLOROPHYLLOUS PLANTS. 87 
which is easily demonstrated, and can be actually measured. 
Even in the annual sunflower, the average height of which 
we may put down as four feet, this root pressure does not 
account for the quantity of water removed from the plant 
by transpiration from the leaves. 
The remarkable fact in connection with root-pressure is, 
that although water ascends to a distance in plants by 
virtue of it, the cell vessels of the root are not themselves 
full of water. In the case of the sunflower, if the leafy top 
were severed during strong sunshine while transpiration 
was rapidly taking place, and a glass tube was quickly 
placed on the root stock and water poured into the tube, 
it would be observed that water was absorbed—sucked 
up—by the root-stock. 
The growth of plants, composed as they are in all parts 
of cells, is chiefly caused by the pressure exerted by the 
cell sap on the cell wall, which helps the cell to increase. 
This condition of the cells is known as turgescence, and as 
the cells only grow and increase when turgescent, the plant 
as a whole only grows when this is the condition of its 
cells. Practically, as every gardener knows, there is no 
growth in a drooping or flacid plant. 
The beautiful net-work of a skeleton leaf must have been 
noticed by everyone; this net-work or venation is of para- 
mount importance. These veins, or vascular bundles, have 
a two-fold function to fulfil: first, they take to the 
assimilating tissue, or mesophyll of the leaf, water laden 
with nutritive matter; and, second, they convey back the 
assimilated products to the various parts of the shoot-axis. 
As the major portion of the organic matter of which 
green plants are composed is obtained from the atmos- 
phere, through the leaves, by the agency of chlorophyll 
contained in them, we will enquire into the organs in the 
leaf which bring about this. 
The most highly organized portion of a plant is the 
