THE NUTKITION OF PLANTS 151 
essential thing is green tissue exposed to light, but in this 
brief account an ordinary leafy plant growing in the soil 
will be considered. 
As the leaves are the active structures in the work of 
photosynthesis, the raw materials necessary must be brought 
to them. In a general way, these materials are carbon di- 
oxide and water. The gas exists diifused through the 
atmosphere, and so is in contact with the leaves. It also 
occurs dissolved in the water of the soil, but the gas used 
is absorbed from the air by the leaves. The supply of 
water, on the other hand, in soil-related plants, is obtained 
from the soil. The root system absorbs this water, which 
then ascends the stem and is distributed to the leaves. 
(1) Ascent of water. The water does not move up- 
wards through all parts of the stem, but is restricted to a 
certain definite region. This region is easily recognized as 
the woody part of stems. Sometimes separate strands of 
wood, looking like fibers, may be seen running lengthwise 
through the stem ; sometimes the fibrous strands are packed 
so close together that they form a compact woody mass, as 
in shrubs and trees. In the case of most trees new wood is 
made each year, through which the water moves. Hence 
the very common distinction is made between sap-wood, 
through which the water is moving, and heart-wood, which 
the water current has abandoned. Just how the water 
ascends through these woody fibers, especially in tall trees, 
is a matter of much discussion, and cannot be regarded as 
definitely known. In any event, it should be remembered 
that these woody fibers are not like the open veins and 
arteries of animal bodies, and no "circulation" is possible. 
These same woody strands are seen branching throughout 
the leaves, forming the so-called vein system, and it is evi- 
dent, therefore, that they form a continuous route from 
roots to leaves. 
It is easy to demonstrate the ascent of water in the 
stem, and the path it takes, by a simple experiment. If 
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