THE PLANT 65 



consider the amount of sap dripping from sugar maples in the 

 spring. It is also noticeable in the watery exudations of some 

 plants after pruning in the spring; especially is this true of the 

 grapevine, where this exudation continues for days and days 

 after pruning. This pressure which forces the sap upward is 

 called root pressure. It is also called osmotic pressure. Scien- 

 tists have found that it is possible not only to measure the amount 

 of water which the roots will raise in a given time, but also to 

 measure the force exerted in the process of root pressure. In 

 the case of the common nettle it is sufficient to support a column 

 of water 15 feet high. With a common vine it will hold a 

 column of water 36.5 feet high, and the birch will hold a column 

 of water 84.7 feet high. This varies for different times of the day 

 and for different seasons of the year. The grapevine shows its 

 greatest pressure in the forenoon and decreases after noon. The 

 sunflower reaches its maximum by 10 A.M. and then begins to 

 decrease. 



Offices or Functions of the Plant. From an agricultural stand- 

 point the plant has four principal uses or functions. 1. It aids 

 in the formation and improvement of soils. 2. It modifies climate 

 and environment. 3. It is the ultimate source of food for farm ani- 

 mals. 4. Plants furnish man with food and other useful materials. 



General Classification of Plants. Plants according to their 

 duration of life may be divided into three classes: annuals, 

 biennials, and perennials. 



Annuals live and die within the year. 



Biennials are plants that live for two years. They grow the 

 first season without blossoming and lay up a stock of nourishment; 

 they blossom and seed the next spring or summer. Turnips and 

 beets are familiar examples. 



Perennials are plants that live and blossom from year to year 

 although some die down to the roots. Our common shrubs and 

 trees may be taken as examples. 



Classification of Crops. When plants are grown in quantity, 

 they and their products constitute a crop. The most important 

 crops may be grouped in four or five classes: 



1. Cereal or Grain Crops, including corn, wheat, oats, rye, 

 barley, rice, etc. 



PRAC. AGRICUL. 5 



