

HABITS OF TREES. 17 



obvious that circumstances or liabilities of this kind may ma- 

 terially modify the effect of chemical applications made to our 

 crops, and may be the often unsuspected cause of important 

 discordancies in our results. 



I might give many other illustrations of the general habits 

 and analogies of our commonly cultivated crops, and quote 

 many special physiological facts, such as that dry weather makes 

 roots like mangold-wurtzel run prematurely to seed, and that 

 the seed so prematurely formed produces plants which, under 

 any circumstances of weather, exhibit a similar tendency, 

 (Stephens ;) that, to succeed equally, some seeds, like that of the 

 parsnip, must be sown new or fresh, (Le Couteur,) while others 

 will germinate readily and healthily though kept for years, and 

 so on ; but the examples already given are sufficient to show 

 that much other knowledge besides what is purely chemical is 

 necessary to the suggester of agricultural experiments even of 

 a chemical nature. His skill in regard to the circumstances in 

 which they are likely to succeed, and therefore ought to be 

 tried, and, above all, his ability to account for failures and dis- 

 cordant results, will in a great measure depend upon the pos- 

 session of this practical physiological knowledge. 



8. So in experiments upon trees, no less than upon field 

 crops, practical knowledge of a similar kind is most necessary. 

 That the clays of the gault and weald favour the oak ; that the 

 elm flourishes only on the soils of the intermediate more sandy 

 strata ; that our cider counties rest chiefly on the old red 

 marls, those of France on the chalks of Normandy, and the 

 tertiary or more recent drifts which overlie them; that, in 

 Bermuda, the coffee-tree grows luxuriantly on the recent hard 

 calcareous rock of that island : such facts as these, with which 

 the practical man is usually most familiar, are all of much use 

 to the experimental adviser, and are rich in suggestions as to 

 the kind of experiments which are likely to succeed upon each 

 species, as to the method of making them, and as to the kind of 

 soils on which good results are to be expected. 



7. Of what the soil consists. 



What has been said of the composition of the plant applies, 

 with some modification, to the soil. 



