6 PLANT LIFE 



the soil is very dry growth is retarded, and in extreme cases the 

 plant may actually lose as much or more than it receives. Then 

 it will wilt and may die. Rainfall or irrigation will dilute the plant 

 food in the soil and thus rapidly increase the osmotic action through 

 the roots. Growth is thus greatly increased. Try an exercise as 

 described under figure 4. 



Plant Food. — The plant requires a number of elements which 

 it gets from the soil, such as nitrogen (N), phosphorus (P), potas- 

 sium (K), calcium (Ca), sulfur (S), iron (Fe). 



The first four of these are most likely to become deficient in any 



Fig. 3. 



Fig. 3. — Root hairs form on all young roots except at the tip. They greatly increase the 

 absorbing surface of the roots. 



Fig. 4.— Osmosis apparatus. A funnel tube with mouth closed by an animal membrane 

 then filled with thick syrup and suspended in water. 



garden or farm soil. Usually they must be replenished by the 

 grower. 



Other elements required by the plant that are taken largely 

 from the air, as already described, are oxygen (O), carbon (C), and 

 hydrogen (H) . Really the oxygen and hydrogen are taken chiefly 

 in the form of water (H2O) which enters the soil from the air and 

 is then taken up by the roots. 



Evaporation from Leaves. — As the plant food from the soil 

 must be very dilute at the time it is taken in by the roots, it is 



