78 



In plantations in high altitudes, however, it must further be taken into 

 consideration that tlie elevation acts differently according as it presents iso- 

 lated peaks or high plateaux. Since the earth's illumination and radiation 

 have considerable influence on the temperature of the layers of air covering 

 it, vegetation at equal heights is exposed to very diverse temperature fluctu- 

 ations. On the high plateau the decrease of warmth with elevation is less, 

 when the sun shines, than on the mountain peak which stands alone. If, 

 however, the sun disappears and radiation becomes determinative, then the 

 lower air layers above the high plateau also cool off more. Thus the daily 

 fluctuations in temperature are much greater here and the seasonal ones as 

 well. On high plateaux the temperature can fall, even to frost, while the 

 isolated peaks remain protected. The same relation is shown between valley 

 and heights ; we have recently observed a number of examples from Italy. 

 Passerini makes^ the following observations from the neighborhood of Flor- 

 ence and cites, as an especially good instance, the night of April 19-20, 1903, 

 when the temperature, which on the 15th still showed +i8.3°C. sank to 

 — i.i"C. and rose again, nine hours later, to -\-i2.2°C. While the vegetables 

 and grains were not injured, the leaves and blossoms were seriously frozen. 

 Only 50 m. higher the injuries were no longer noticeable. 



In mountainous regions clouds and mist act as a protection from frosts. 

 Thomas- observed in Thiiringen that the young beech foliage did not 

 suffer from frost at heights covered by mists while in the valleys and gorges 

 the leaves were injured. The artificial prevention of frost by the production 

 of smoke has been founded on the peculiarity of mists which prevents the 

 sharp fall in temperature. 



Adjustment of the Root Body of Woody Pl.\nts. 



In mountains the adaptation of the wood body to the rocky soil and the 

 compensatory structures which appear on this account are especially interest- 

 ing. In the following figure i, we see the root of an oak which has made 

 its way through a fissure in a rock and by its continued growth in thickness 

 within the split has developed into a flattened, board-like form. After leav- 

 ing the rock, the root resumed its cylindrical form. This example shows 

 first that, despite the pressure which the strong root had withstood for so 

 many years, the ability to conduct water and plastic material has not been 

 interrupted in the board-like part. In the second place, we notice above the 

 board-like flattening the appearance of adventitious roots. Both processes 

 correspond to the phenomena caused by artificial constriction. 



So far as we have been able to investigate roots which had been flatten- 

 ed in the clefts of rocks, we could observe that the board-like flat places in 

 the root body were produced because the wood rings formed every year were 

 very strongly developed on the sides where they could develop freely. 



1 Passerini, Sui danni prodotti alle piante del shiacciato etc. Bull. Soc. Bot. 

 ital. 1903. p. 308. 



2 Thomas, Fr., Srharfe Horizontalgrenze der Frostwirkung- an Buchen. Thiir. 

 Monatsblatter. April 1904. 



