_Germination of Seeds._—While wild plants of special adaptation may thrive in very low (or high) temperatures, it is also true that few of our cultivated plants will germinate, and still less grow thriftily, at such low temperatures. The limit below which most cultivated plants may be considered as remaining practically inactive lies between 4.4 and 7.2° C. (40 and 45° F.). Few tropical plants will germinate much below 23.8° (75° F.) and in some cases not below 35° Cent. (95° F.). Even maize and pumpkins, according to Haberlandt, germinate most rapidly between 35 and 38.3° C. (95 and 101° F.), while for wheat, rye, oats and flax the best temperature for germination lies between 21.1 to 26.1 (70 to 79°). Under the most favorable conditions of temperature and moisture, some small seeds which readily absorb moisture will germinate in from twenty-four to forty-eight hours, while at a lower temperature they may require from three days to two weeks. Thus Haberlandt found that while oats would germinate in two days at a temperature of 17.2 to 17.5° C. (63° to 63.5°), it took a full week for germination when the temperature was only 5° C. (41° F.). It is obvious that seeds remaining inert in the soil for such lengths of time will be subject to a variety of vicissitudes that may injure or destroy their vitality. There are many bacteria and fungous parasites which at low temperatures are perfectly capable of attacking and destroying the water-soaked seed. There is thus for each plant, from the lowest to the highest, a certain temperature most favorable to development; and both above and below this, the vegetative activity is seriously interfered with or wholly checked. A knowledge of these limits is manifestly of the utmost practical importance. The influence of too high a temperature in preventing the germination of cinchona seed from India, was curiously exemplified when it was subjected to a supposedly favorable steady temperature of 23.8° C. (75° F.) under otherwise most favorable conditions. Not a single one came up in the course of six weeks, and the box in which it had been sown was put away outside of the hothouse as a failure. Within two weeks a full stand of seedlings was obtained, at temperatures ranging between 12.7 and 15.5° C. (55° and 60° F.). The fact that the cinchona is a tree of the lower slopes of the Andes (three to five thousand feet) although at home strictly within the tropics, explains the apparent anomaly. PART THIRD. CHEMISTRY OF SOILS. CHAPTER XVIII. THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP PRODUCTION. The chemical constituents of soils have been incidentally mentioned and discussed above, both in connection with the processes of soil-formation, and with the minerals that mainly participate therein. The manner of their occurrence and their relations to plant life, so far as known, must now be considered more in detail.