36o 



foundation in the fact that these are dry years, favoring the maturing of the 

 blossom buds. 



Even if twig abscission is of httle practical importance in forestry, 

 it is, however, of horticultural importance as a symptom. Especially in 

 the autumn the stem parts of many greenhouse plants are abscissed, as in 

 the bushy Begonias, Melastomaceae, Acanthaceae, etc. They are positive 

 indications of excess of water, and the only means of prevention is to keep 

 the plants dry. 



b. Increase of Food Concentration. 



Among the phenomena of disease to be discussed in this section, those 

 must be considered in which an excess of water in the plant becomes mani- 

 fest locally. In this the root activity is not necessarily increased, the accum- 

 ulation of water is produced rather by a depression of the transpiratory 

 activity of the leaves. Increase in turgor must set in in various organs, or 

 parts of organs, by increased water supply, as has been proved artificially in 

 severed leaves. Consequently, the fact remains to be considered here that 

 the humidity of the air often co-operates decisively. Conversely, in other 

 cases, in which an excess of nutrients is involved, attention should be called 

 to the fact that this excess does not always presuppose an absolute accum- 

 ulation in the soil, but also occurs when the solvent, i. e., the water, is 

 temporarily present in too small an amount, thereby producing an injuriously 

 high concentration of the soil solution. 



The demands made upon the soil solution by each species seem to dift'er 

 according to the different cjuantitative proportions in which the various 

 nutrients and other factors of growth participate in the production of one 

 gram of dry weight of a species. In plants, for example, which require 

 much potassium or nitrogen to produce their substance, a high percentage 

 solution of these substances will be necessary for the root. The plants do 

 not die, if the desired high concentration is not afforded them, but they 

 change their mode of growth. They then require, as already proved, much 

 more water just as if they must strive to obtain the necessary quantity of 

 a certain nutrient by an increased absorption of the more dilute solution. 

 In spite of the large quantity of water and substances otherwise offered, 

 the production as a whole is small. A similar cessation of growth is found, 

 if the plants are placed in a too concentrated soil solution. The absorption 

 of water is relatively scanty; the amount of ash, however, large and the 

 production in dry weight small. The excess then is taken up but not uti- 

 lized, the mineral substances are simply deposited in the plant and may 

 partially be leached out again by water. In water cultures with a high 

 concentration of nutrients the short, gnarled root hairs are sometimes per- 

 ceptibly covered with crystalline scales. Thus, for example, accumulations 

 of saltpetre may take place in the plant if an excess of potassium nitrate is 

 given. Emmerling\ by means of experiments, explains very acceptably 



1 Emmerling-, A., Beitrage zur Kenntnis der chemischen Vorg-ange in der 

 Pflanze. Landwirtsch. Versuchsstationen, Vol. XXX, Part 2, 1884, p. 109, 



