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THE AMERICAN BOTANIST 



Books on the Iris. — The publication of the article on the 

 iris in the February number of this magazine has brought out 

 several requests for further information regarding this inter- 

 esting group of plants. The literature, however, does not seem 

 to be abundant. The matter relating to the irises in gardening 

 books has reference largely to those species that are commonly 

 cultivated and about which the beginner is likely to be already 

 informed. The only book dealing exclusively with irises is 

 Irwin Lynch's admirable ''Book of the Iris" which costs a 

 dollar. C. S. Harrison, York, Nebr., has issued a 30-page 

 'Tris Manual" which deals with the cultivation of these plants 

 and though much less extensive than Lynch's volume will be 

 very useful to beginners. It costs 25 cents. 



Limiting Factor in Soils. — Only ten chemical elements 

 are said to be indispensable to plants. All the sugars, starches, 

 vegetable oils and wood fibres in the world are made from three 

 of these, namely carbon, hydrogen and oxygen. They are 

 literally "made of wind and water" for the necessary elements 

 are derived from the air and the moisture in the soil. The other 

 elements are needed for building up more complex substances, 

 including protoplasm the living matter of the cell. Some are 

 needed to perfect seeds, others in changing energy into useful 

 work and still others tO' neutralize harmful acids in the plant. 

 This accounts for the fact that plants that are depended on to 

 supply the substances mentioned above, must have more than 

 the three elements to work with and a single missing element 

 sometimes makes a great difference in the size of the crop. 

 Most soils are easily depleted of several of the chemical ele- 

 ments needed by plants. This is the reason why farmers and 

 gardeners spend so much thought on methods of enriching the 

 soil. Usually the element least abundant is nitrogen but 

 potassium and phosphorus are frequently as scarce. In some 

 experiments made at the University of Illinois it was found that 

 wheat fields yielding only about six bushels of wheat to the acre 



