35 



ature, rainfall, composition of the soil, are so variable and indetermi- 

 nate as to render them of comparatively little scientitic value. Excep- 

 tions to this rule, however, are the remarkable experiments of Lawes 

 and Gilbert, in which the same crops have been raised year after year 

 on the same land, each plot receiving' continuously the same manure. 

 By thus continuingthe experiments through a series of years, the sources 

 of error are compensated and results of the highest importance obtained. 

 The results of the experiments of Lawes and Gilbert have already been 

 made known to the English reading public, and we forbear any descrip- 

 tion here. 



2d. Experiments for raising plants in artificial soils and solutions, in 

 order to determine what are the nutritive ingredients of the food of 

 I)lants and what are their functions in nutrition. A description of the 

 character and results of investigation in these subjects, up to the year 

 1869, may be found in the admirable books of Johnson, " How Crops 

 Grow" and " How Crops Feed." We append descriptions of some later 

 researches. 



The important questions of the relation of the atmosphere and soil 

 to the nutrition and growth of plants have been the subject of extended 

 research in the experiment stations and elsewhere. Of special interest 

 are the subjects of the relation of the nitrogen of the air to vegetation ; 

 the formation of nitrogen compounds in the soil, or nitritication, and 

 the absorptive power of soils, which have also been treated most satis- 

 factorily in the works referred to. 



Experiment stations for investigations in vegetable phytsiology. — Among 

 the stations where the most work in this line is done are those at Thar- 

 and, in Saxony ; Dahme, in Prussia ; and Hohenheim, in Wiirteinberg. 

 A good example of these is the one at Tharand, near Dresden, in Saxony, 

 •whose object as enunciated in a resolution of the Agiicultural Society by 

 which it w«is founded in 18G9, is, " to investigite scientifically the 

 natural laws by which theculturjB and utilization of the plants important 

 in agriculture are governed." (V's.-St., XI, 1869, p. 224.) 



The investigations made thus far have been, as stated by the director, 

 Dr. Nobbe, applied to "the study of tlie role which the mineral ingre- 

 dients of plant-food play in the vital processes of cultivated plants." 

 (Vs.-St., XIII, 187i; p. 321.) 



A brief notice of some of the later results of previous research may 

 facilitate an understanding of the objects, methods, and results of the 

 Tharand investigations. 



It is only within a very few years that any reasonably exact knowl- 

 edge has been atrained as to what ones of the mineral elements found 

 by analysis to exist in plants are essential to their normal growth, and 

 what ones are accidental or superfluous. 



Some of the most important data for the solution of these questions 

 have been obtained by what is known as " water-culture." This con- 

 sists in raising plants not in soil, but in water containing in solution 

 the substances needful for their nutrition. 



It is known that plants take up from the soil through their roots cer- 

 tain mineral matters, as lime, potash, silica, and phosphoric acid. In 

 order to learn which of these are indispensable to the growth of the 

 plant, an obvious plan would be to supply to the plant, during its growth, 

 the ingredients it is known to take from the soil, varying the number 

 and quantities of these suitplied in ditferent exj)eriments, and noting 

 the results. Solution in water affords a most satisfactory means of 

 regulating these sup{)lies. Seeds ar»^ therefore allowed to germinate 

 in sand or moist cotton, and then suspended so that the roots may 



