134 BOTANY FOR BEGINNERS CHAP. 



temperature by placing in an old tin and applying heat to it. Sow a few 

 Clover seeds in each ; give to (a) water only, but to (b) give the solution 

 described on page 118. From time to time pull up a plant from each 

 pot and examine. Note 



(i) The difference in the size of the plants. 



(ii) The difference in the root-tubercles. 



Remaining Elements in Plants. Sulphur. Plants 

 take in sulphur in the form of sulphates. The sulphates of 

 ammonia, potassium, and calcium are the most useful to the 

 plant. Sulphur enters into the composition of proteids and 

 protoplasm, and without it these materials cannot be formed. 



Phosphorus. Plants take in phosphorus as phosphates. A 

 common phosphate is calcic phosphate, or phosphate of lime. 

 Phosphorus enters into the composition of the nucleus, and 

 appears to aid those chemical changes upon which the life of 

 the plant depends. 



Potassium. There is a very large variety of forms in which 

 potassium can be absorbed by plants, such as sulphates, 

 phosphates, and chlorides. As a rule clay soils possess plenty 

 of potassium, and it is very seldom that a compound of potassium 

 is applied as a manure. This element is very active in assimila- 

 tion and in the formation of protoplasm. The solid matter of a 

 plant contains as much as 3*5 per cent, of potassium. 



Calcium. The calcium which a plant requires for its growth 

 is absorbed in the form of sulphates, phosphates, or nitrates. 

 The work which calcium plays in the economy of the plant is 

 not fully understood. Plants cannot live without it, and 

 the effects of an insufficient supply is shown by their retarded 

 growth. 



Magnesium. Magnesium can be taken in by plants from all 

 its compounds except the chloride, which seems to be injurious. 

 Very little is known as to the use of magnesium, but our experi- 

 ments in water culture show that it is necessary for the healthy 

 growth of the plant. 



Iron. Green plants, as we have seen, require iron in their 

 food for the formation of chlorophyll. This element can be 

 absorbed from a variety of compounds, and it is only an 

 essential element for the nutrition of green plants. 



The Non- Essential Elements of Plant Food. 

 A very large number of elements which plants take in with 

 their food they can do without. 



