FOOD OF PLANTS. 117 



influence. AVe believe, however, more depends on the action of 

 elements of the atmosphere and light than upon the neceasarilv 

 limited time of suspension of vitality in the seed itself. We have 

 known .seeds, which are generally considered as losing their 

 vitality at the end of one year, readily germinate after being 

 kept for several years in a dry atmosphere, and of nearly uni- 

 form temperature, and protected from the light. Seeds which 

 are generally considered as retaining their vitality only one year, 

 have been known to germinate at the end of one hundred years, 

 and cases are often recorded of seeds germinating after lying 

 buried in the earth beyond the reach of the atmosphere for at 

 least seventeen hundred years. Any table on this subject, there- 

 fore, is necessarily a very imperfect guide, unless it should be 

 based on some specified manner of keeping the seeds. The best 

 course to follow in the preservation of seeds, is to keep them as 

 much as possible in a temperate and dry atmosphere, protected 

 from the action of the light. The cotfee-seed has never been 

 made to germinate, uidess planted immediately on its ripening. 

 Oily seeds soonest lose their vitality. In the purchase of seeds, 

 kept in the usual manner, fresh ones should always be required. 



Section 4. — Food of Plants. ^^^ 



211. The principal food of plants is water and carbonic acid 

 and ammonia, which are received through the roots in a liquid 

 state, and through the leaves in a gaseous form. Besides these, 

 various salts enter in a greater or less degree into the composi- 

 tion of vegetables. 



To determine the food of plants, it is an important element in 

 the investigation to know of what the plant is composed. This 

 has been determined by various philosophers with great accu- 

 racy. The following is the constitution of some of the most 

 common plants, taking 1000 parts of the dry vegetable : 



Carbon. Hydrogen. Oxygen. Nitrogen, Ashes. 



Wheat 455 57 430 35 23 



Oats 507 64 367 22 40 



Peas 465 61 401 42 31 



Turnips 429 56 422 17 76 



Potatoes 441 58 439 12 50 



Quite a uniformity will be observed in these elements ; about 

 one half being carbon, less than half oxygen ; about one- 

 twentieth hydrogen, less than one-twentieth nitrogen, with a 



The best conditions of keeping them ? — 211. The principal food of phmts ? 

 What is important to know to determine the food of plants ? Mention the 

 constitution of some plants. 



