28 THE PLANT. 



shooting and flowering, the root was rich in soil con- 

 stituents ; after the production of seed, its store of them is 

 exhausted. If the plant is cut off and the root left in the 

 ground, before flowering, the soil retains the far greater 

 portion of the nutritive matter which it had given to the 

 plant ; on the contrary, after flowering and the production 

 of seed, the root retains only a small residue of these con- 

 stituents, and the soil is correspondingly exhausted of 

 them. 



As it is with the turnip, so is it with culmiferous plants. 

 If they are cut off before flowering, a considerable portion 

 of the nutritive substances stored up in them remains in 

 the root, which the soil of course loses, if the overground 

 plant is removed after the ripening of the seed. 



The experience derived from the cultivation of tobacco 

 gives a clear view of the processes in the develop ement of 

 an annual leafy plant. 



In the tobacco plant the overground and the under- 

 ground parts grow with perfect equality ; the root gains 

 in extent, in the same proportion as the stem lengthens 

 and the leaves increase in number and size. There is no 

 appearance of sudden changes in the direction of organic 

 activity, no shooting, but the phases of life in the plant 

 follow in steady continuous progression. Even while the 

 top of the stem bears ripe seeds, and the lower leaves 

 have withered, the side shoots of the plant are often still 

 putting forth flower-buds, the seeds of which will ripen at 

 a much later period. 



The tobacco plant is remarkable for producing in its 

 organism two nitrogenous compounds, of which the one, 

 nicotine, contains neither sulphur nor oxygen ; while the 

 other, albumen, is identical with the sulphureous and 

 oxygenised constituents of the cereals and other alimen- 

 tary plants. 



