THE ELEMENTS NEEDED BY PLANTS 97 



the simple expedient of planting such water-loving species as 

 willow and cottonwood in the vicinity. A great part of the 

 living plant is water. Turnips, melons, and the like contain 

 more than 90 per cent of water, and even in air-dry plants the 

 amount is seldom less than 10 per cent. The amount of water 

 may differ greatly in different parts of the same plant ; thus 

 the flesh of the watermelon, peach, plum, and the like contain 

 much more water than the seeds or the vegetative parts of 

 the specimen. 



Root pressure. Most of the water given off by plants escapes 

 through the stomata as water vapor, but occasionally, as at the 

 close of a warm day in summer, the roots may continue to absorb 

 more than the leaves can evaporate in the cool air of evening. 

 This excess moisture may appear on the leaves as minute glob- 

 ules or even larger drops of water. Such excretion of water 

 is called guttation, and the force exerted by the roots in send- 

 ing it upward is root pressure. In some plants this force is very 

 great. In the birch it is sufficient to hold up a column of water 

 more than eighty feet high. It is root pressure that causes 

 grapes to " bleed " when trimmed in spring, and the same 

 force makes the sap run from wounds in trees. The drops of 

 water to be seen on the leaves of such plants as nasturtium in 

 the early morning are due to root pressure, and so is much of 

 what passes for dew on grassy areas. Often the roots of weeds 

 cut down by the hoe will continue to send up water for some 

 time and show, by a moist spot in the dry surface soil, where 

 each plant stood. 



Carbon dioxide. The gas, carbon dioxide, though found in 

 the atmosphere in so small an amount as three parts in ten 

 thousand, is nevertheless the only source of the carbon in 

 plants. In a ton of dry wood at least a thousand pounds is 

 carbon, all of which has been derived from the air. The carbon 

 in our hard and soft coals, peat, and the like was stored up in 

 the same way and from the same source in other days. This 



