82 PLANT PHYSIOLOGY 
the same. Frequently, however, they are changed chemi- 
cally after they enter the plant and then are no longer able 
to pass through the external plasma membrane. In 
such a case the plant may be able to take in large amounts 
of one substance from a dilute solution. Certain sea- 
weeds, for example, accumulate large amounts of iodine 
compounds from the sea water which contains iodides 
only in very great dilution. 
119. Water consists of hydrogen and oxygen (H,O). 
Besides these two elements eight. others are ordinarily 
necessary to plant life. They are carbon (C), which 
chiefly enters the plant in the form of carbon dioxide 
(COs) (see paragraph on photosynthesis), nitrogen (N) 
in the form of nitrates or ammonium salts, calcium (Ca), 
magnesium (Mg) and potassium (K), these mostly oo- 
curring as phosphates, nitrates, sulphates or carbonates, 
iron (Fe) in very small amounts as salts of various acids, 
sulphur (S) almost entirely as sulphates (except in those 
plants that feed on organic food where it may be taken up 
from the proteins and a few lower plants which use 
H.S or even free sulphur) and phosphorus (P) as various 
phosphates. In addition to these, sodium (Na) is re- 
quired by some plants, while on the other hand calcium 
(Ca) is not required by certain fungi. Of the ten 
elements first mentioned the last seven are usually taken 
in as mineral salts from the water in which they are 
dissolved. ‘The oxygen is taken in, in the acid radical of 
the sulphates, nitrates, carbonates and phosphates, in 
combination with hydrogen in water, and in combination 
with carbon in carbon dioxide as well as in the elementary 
form directly from the air or in solution in the water. 
Carbon in addition to being taken in as carbon dioxide 
exists in the carbonates and in the case of hysterophytes, — 
also in various organic substances taken in by the plant. 
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