123 



Wittmack has reviewed earlier cultural experiments as to the behavior 

 of plants indigenous to any given locality when artifically introduced to a 

 region farther souths His conclusions follow; — plants from the north de- 

 velop somewhat more slowly in middle Europe, catch up later with the in- 

 digenous ones, however, or even exceed them. It is evident, therefore, that 

 the short growth period, which has become habitual in the north, is often 

 still more shortened by the increased w^armth of the southern habitat, pro- 

 vided also that the climate be dry. The damp climate of England with its 

 low^ maximum temperatures retards ripening. The humidity of the air is a 

 1 actor of great power and can delay ripening; just as, conversely, regions 

 with great periods of drought, the climate of the steppes and similar con- 

 ditions, not dependent on the degree of latitude, form limited centres where 

 plants ripen prematurely. Too great drought certainly retards development, 

 as has been determined experimentally. Stahl-Schoder's experiments, cited in 

 the chapter on "Excess of Water," treat of soil dryness. The period of the in- 

 fluence of heat is very important and is indeed explicable. Heat in July and 

 August is more advantageous than in May and June but the reverse is true 

 for rain. 



Wittmack's summary in general shows the significance of the physical 

 structure of the soil in relation to the early ripening; — that the vegetative 

 time in eastern regions is shorter for the same varieties of grain than in 

 western ones. 



Based on the observation that the varieties cultivated in northern 

 climates retain their shorter growth period in the immediately following 

 developmental periods, an active trade in northern seed has been developed. 

 Meanwhile the quantity of the harvest should not be lost sight of. Abundant 

 supply of nutrition being uniformly assumed, the quantity depends always 

 on the length of the vegetative period, — i. e., the time of the formation of 

 shoots. The longer time the grain has for the formation of vegetative 

 organs (as in damp, cool seasons) the more abundant is the grow^th of 

 shoots and with it the formation of a greater number of ears from the in- 

 dividual seeds. 



H we should carry into the east varieties produced in the west, which 

 are long-lived and characterized by great productivity, we would run the 

 risk of frosts. This is most strikingly true in the English varieties of wheat, 

 from the squarehead group, wdiich to\\'ard the east come less and less true 

 to seed, because they winter kill. Experience shows in regard to frost-resis- 

 tance, that seeds from northern regions give plants in southern latitudes 

 which at times not only ripen earlier, in spite of an initial retardation, but 

 also better withstand frost. 



From the result of Schiibeler's^ observations, it should be emphasized, 

 that the quick growth, which has become habitual in northern or Alpine 



1 Ueber vergleichende Kulturen mit nordischem Getreide, Von Dreisch, Kor- 

 riicke, Kraus, Vilmorin and others, referred to by Wittmack. Landwirthsch. Jahrb. 

 1875, p. 479. and 1876, pp. 613 ff. 



- Schiibeler, Die Pflanzenwelt Norwegens, 1873, pp. 77 ff. 



