THE NUTRITION 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 diffused 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 brandling 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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