37 



which we usually pay too little attention. Aside from the fact that the 

 weather of one year always varies from that of the preceding year, the soil 

 too is always dififerent ; indeed partly because the time and method of 

 working as well as the fertilization and previous cropping in themselves 

 always effect changes, and partly because this changed arable land is also 

 subjected to changed weather conditions, so that it diiTers every year physi- 

 cally and chemically for the same variety. In the main portion of the book 

 a sufficient number of examples of the influence of planting, previous 

 cropping, mechanical soil constitution and such factors will be cited and it 

 will be shown how these can influence the character and power of resistance ; 

 as, for example, to frost. 



In the second place we think that the running out of a cultivated variety 

 can also arise because the variety itself changes its character. According to 

 our hypothesis, there is, in all organisms, no stability ; there is no strict ma- 

 terial or formal repetition of any process, because the organism changes in 

 the smallest unit of time, at each moment confronts the same factors of 

 growth as a dififerent organism and strides forward to adjustment. Thus 

 each variety, like every term of relationship or of classification, is only a 

 frame work made up of common characteristics in which individuals con- 

 stantly fluctuate because of lesser variations. 



An excess of nitrogen develops a plant substance ditterent from that 

 produced by moderate nitrogen nutrition, a deficiency of potassium makes 

 an organ dififerent from that grown with an abundance of potassium. Abun- 

 dance of light and deficiency of light develop the cell wall in different ways, 

 great warmth produces more sugar than scanty amounts of heat, etc. Exact 

 examples are given in the chapters on the action of individual factors of 

 growth. Therefore the organism is like wax which, because of the thrusts 

 of the individual vegetative factors, is constantly pressed into other material 

 forms. 



The material constitution of the plant body, however, is changed by the 

 variations of the molecular arrangement which we call chemical changes, as 

 well as by the mechanical ones in which the chemical composition remains 

 constant. The mechanical disposition of water in the tissues, the substances 

 carried in in the water, the tension conditions in the cell wall and the cell 

 contents, are all factors which change constantly and as constantly influence 

 each other dififerently. The slightest increase in the supply of light is a 

 thrust which not only influences the assimilatory process, but must also in- 

 directly exercise an effect on all other functions. This does not depend at 

 all upon whether we can define these effects ; — the proof that they must take 

 place is enough. 



Let us now consider how the thrusts of individual factors of growth act 

 normally on the plant body. Here we notice a peculiar alternation. At day- 

 break the action of light begins ; — assimilation, evaporation, thickening of the 

 cell wall etc. are increased, the whole structure reflects all the phenomena of 

 the light reactions. At nightfall, when the after eft'ects of the light have 



