78 ELEMENTARY BOTANY. 



in the perforated cork, with cotton, a seedling that has a radicle a half 

 inch long or more so that it will grow down into the water. The seed 

 itself should not touch the water. Surround the vessel with a paper jacket 

 to exclude the light in order to prevent the growth of Algae. Set in the sun- 

 light and replace the fluid from time to time as necessary. The entire solu- 

 tion should be renewed every two weeks. It has also been recommended to 

 place the plant from time to time in a saturated solution of gypsum and to 

 aerate the liquid frequently by an aspirator, but this is not necessary for 

 ordinary demonstration. 



8. Both by analysis and by cultures it has been determined 

 that the following elements are necessary to plant life : carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, iron and sodium. Carbon constitutes 

 usually about one-half of the entire dried substance of the 

 plant. This element is derived mainly from the carbon-di- 

 oxide (COj) of the air. This gas, ordinarily about four hun- 

 dredths of one per cent, of the atmosphere, is taken through 

 the stomates of the chlorophyll-bearing plants, and in the pres- 

 ence of sunlight decomposed, the carbon being retained and 

 the oxygen set free or to some extent separately consumed. 

 Plants which do not contain chlorophyll get their carbon 

 from absorbed organic compounds which have this element 

 as one of their constituents. The plants that have chloro- 

 phyll — while obtaining nearly all of their carbon from the 

 air — are nevertheless capable of absorbing complex carbon 

 compounds. For example, " insectivorous " plants absorb 

 such substances, and these are known to be important factors 

 in their nutrition. 



9. The element hydrogen constituting a very much smaller 

 per cent, of the substance of the plant than carbon, is derived 

 mainly from water (HjO). A portion is derived from ammonia 

 (NHs) and its compounds and from complex carbon com- 

 pounds. Oxygen forms the largest portion, after carbon, of 

 the weight of the dried vegetable substance. It is taken into 

 the plant both in the free state and in combination. The free 

 oxygen of the air is absorbed (respired) by all plants and is 

 largely concerned in the destructive processes in nutrition. 

 The oxygen that is specially concerned in the constructive 



