38 



ceased, processes of oxidation come to the front, phenomena of increased 

 turgidity, conversion of starch and the Hke. The same changes may be ob- 

 served in the media surrounding the plant, in the air and soil. A decrease 

 of warmth and increase of water content must act powerfully on the plant 

 body. With the change between day and night is associated the influence of 

 the seasons, which forces upon the plants a period of rest after a time of 

 production. Therefore we find in nature a "corrective pcriodicUy." Amid 

 these regularly alternating fluctuations of the vegetative factors, the plant 

 balances its growth and completes its normal course of development. 



Since the duration and action of these periods in each year differ, the 

 production of each plant differs also and the individual years are thus char- 

 acterized. We speak of dry and wet years and know from experience that 

 in the former, the yield of grain is noticeably large, while the straw yield is 

 less on account of the shortness of the stalks. In wet years this is reversed. 

 And although the farmer then complains that the baking quality of the flour 

 lias suffered, yet he emphasizes the fact that he finds compensation in the 

 greater straw harvest. 



This example, taken from general practice, shows how great single vari- 

 ations in the average periodicity at once becomes noticeable since the prefer- 

 ence is shown for different peculiarities of the plant body. As long as this 

 kind of one-sideness in development does not threaten the existence of the 

 individual plant we may leave the results out of consideration and seek to 

 equalize possible cultural variations (as, for example, by the crossing of 

 grains possessing poorer baking qualities with those rich in gluten which 

 come from dry, warm regions). 



However, the single prevalence of a definite atmospheric factor can also 

 lead to direct disease since the effects are cumulative. Such an accumulation 

 of effects may be compared to the increase in celerity in falling bodies where 

 the distance of the fall equals the square of the time. If, instead of gravity, 

 we assume another factor, such as v/et, cloudy weather, it will in one day in- 

 crease the water content of the tissue while the wall thickening remains be- 

 low normal. On the second day, the first day's action is doubled and the 

 already porous tissue becomes still more porous. The thrust against the 

 plant body, which in itself would not produce disease, is cumulative to an 

 extent ultimately threatening the plant's existence. Practically we find this 

 even within one vegetative period, as, for example, lodging of the grain in 

 rainy seasons. The moisture has lengthened consideral)ly the cells at the 

 base of the stalk while the deficiency of light has essentially arrested the 

 thickening of their walls. The result is that the weakened base is not able 

 to sufficiently resist the strain of the wind and gives way. The development 

 of the grain is weakened or inhibited, according to the extent of this lodging 

 and the phenomena resulting from it, so that the stalk iself is also brought 

 to a premature death. 



Corresponding to the above mechanical changes in the wall, the cell 

 contents are subjected to changes leading to a diseased condition in the case 



