THE NUTRITION OF IMjANTS 151 



essential tiling is green tissue exposed to light, but in this 

 brief account ;ui ordinary leal'y jilant growing in the soil 

 will be considered. 



As the leaves are the active structures in tiie work of 

 pliot(jsynthesis, 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, Ijut the gas used 

 is absorbed innn the air liy the leaves. The supply of 

 water, on the other hand, in soil-related plants, is oljta.ined 

 from the soil. The root system absorbs this water, which 

 then ascends the stem and is distributed to the leaves. 



(1) A.scciit of water. — The water does not move up- 

 wards through all j)arts of the stem, but is restricted to a 

 certain definite region. This region is easily recognized as 

 the Avoody })a.rt of stems. iSometimes separate strands of 

 wood, looking like fibers, may be seen running lengthwise 

 through the stem ; sometimes the iibrous strands ai'c ]iacked 

 so close together that they form a compact woody mass, as 

 . in slii'ul:is and trees. In the case of nujst trees new wood is 

 made each year, throngli which the water moves. Hence 

 the very common distinction is made between sap-?i'ood, 

 through whicli the water is nujving, and /leaii-ii'dnd, Avhich 

 the water current has aljandoned. Just liow 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 

 tliat these woody filjers a.re iu:)t like the open veins and 

 arteries of animal bodies, and no " circulation" is possiljle. 

 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, Ijy a simple experiment. If 

 11 



