138 



ROOTS AND UNDERGROUND STEMS 



bustible material ? What proportion of the solid material 

 was destroyed by combustion ? 



Test in the same way the fresh, active parts of any kind 

 of ordinary land plant (sunflower, hollyhock, pea vines, etc., 

 make good specimens) and of some kind of succulent water 

 or marsh plant (Sagittaria, water lily, fern, etc.). Do you 

 notice any difference in the amount of water given off and 

 of solid matter left behind ? In the character of the ashes 

 left.' Have you observed in general any difference be- 

 tween the ashes of different woods ; as, for instance, hick- 

 ory, pine, oak, etc. ? 



197. Essential Constituents. — The composition of the 

 ash of any particular plant will depend upon two things : 

 the absorbent capacity of the plant itself and the nature 

 of the substances contained in the soil in which it grows. 

 But chemical analysis has shown that however the ashes 

 may vary, they always contain some proportion of the fol- 

 lowing substances : potassium (potash), calcium (lime), mag- 

 nesium, phosphorus, and (in green 

 plants) iron. These elements occur 

 in all plants, and if any one of them 

 is absent, growth becomes abnormal 

 if not impossible. 



The part of the dried substances 

 that was burned away after expelling 

 the water consists, in all plants, 

 mainly of carbon, hydrogen, oxygen, 

 nitrogen, and sulphur, in varying 

 proportions. These five rank first 

 in importance among the essential 

 elements of vegetable life, and with- 

 out them the plant cell itself, the 

 physiological unit of vegetable struc- 

 ture, could not exist. They compose 

 the greater part of the substance of 

 every plant, carbon alone usually 

 forming about one half the dry 



4 2 1 J 5 



283. — Water cultures of 

 buckwheat, showing effect 

 of the lack of the different 

 food elements : i, with all 

 the elements ; ■^, without 

 potassium ; 3, with soda 

 instead of potash; 4, with- 

 out calcium; S, without ni- 

 trates or ammonia salts. 



