HORTICULTURAL JOURNAL. 145 



During the great heat of summer, ammonia may occasion accidents. It 

 would be well therefore, to suspend its use during the months of June, July, 

 and August. Such accidents as have been observed always occurred under 

 the same conditions, and were of a uniform character. They aifect chiefly 

 plants whose vegetation is far advanced. The leaves turn yellow, wither, 

 and fall; even though the atmosphere may be saturated with moisture, the 

 evil extends to a certain number of leaves at the top, and the plant dies. 

 This effect is the result of a certain failure of equilibrium between |the 

 quantity of the elements absorbed by the leaves and the roots. It is 

 through the roots that mineral substances are supplied to plants. If the 

 absorption of these substances goes beyond a certain limit, the plants can- 

 not use all that they receive and they form a saline efflorescence on the sur- 

 face of the leaves. If after a heavy rain the weather becomes dry, we ob- 

 serve frequent examples of this sort of efflorescence upon the large leaves 

 of cucurbitacere. 



"When under different circumstances, the activity of the leaves exceeds 

 that of the roots, the absorption of organic elements becomes predominant. 

 For want of a sufficient supply of mineral matter, these elements cannot be 

 usefully appropriated. Then a remarkable effect is observed, that which the 

 roots cannot yield to the plant it obtains within itself; and there is a re-ab- 

 sorption of the mineral substances of a certain number of leaves. In na- 

 ture we often see examples of this re-absorption of the older organs to the 

 advantage of those more recently formed. If we break off a plant of Purs- 

 lain when it is in flower and put it on a sheet of paper in the shade, the 

 vegetation continues, the seed forms and ripens. Now in this case the min- 

 eral substances contained in the seed could not be derived directly from the 

 soil, but must therefore have been drawn from the tissues of the plant it- 

 self. 



The following conclusions may be drawn from the observations above de- 

 tailed. 



First — In the proportion of 4.10,000' ammonia added to the air im<part& 

 to vegetation a remarkable activity. 



Second — A given weight of the product thus obtained contains more ni- 

 trogen than that of the same plant grown in the pure air. 



We may add, that periods may be selected for the use of ammonia in 

 which its influence causes very different effects. 



First — If we commence the use of ammonia two or three months before 

 the flowering of the plant, vegetation follows its usual course, and no dis- 

 turbance takes place in the successive phases of its growth. 



Second — If we begin to supply the gas at the moment of flowerings the 

 formation of flowers is arrested or retarded, the plant covers itseli" with 

 leaves but produces no fruit. 

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