SOIL EXHAUSTION AND ROTATION OF CROPS. 151 



should fortify it considerably against this strain, and a small 

 seed, which renders the exhaustion less than is the case with 

 our bread-grains. The production of seed is thus a critical 

 thing for the life of the plant. 



Let us consider again for a moment, the mode of growth 

 of our cereal grains. Sown in the spring, the plant comes up 

 and grows, slowly at first but with increasing vigor, up to the 

 time of "heading out" — a period of two months. Then the 

 growth acquires its greatest intensity. It heads out, blossoms, 

 and the seeds begin to form and ripen, and tiiis vv^hole pro- 

 cess of seed-production requires but about a month when the 

 weather is favorable for its completion.. 



In actual trials with the oat plant, it has been found by 

 Bretschneider and Arendt that a large share of the growth of 

 the over-ground part of the plant occurs at the time of head- 

 ing and blossom. Thus the former observed that out of 6,886 

 lbs. of the dry acreage yield of the oat, 3,099 lbs., or three- 

 sevenths of the crop, were produced from June 19th to July 

 8th, i. e., in nineteen days; the total period of growth being 

 one hundred and six days. Arendt found that three-eighths 

 of the total dry produce of the oat grew in twelve days, 18th 

 to 30th of June, the period of heading and bloom, and dur- 

 ing the twenty-two days between June 18tli and July 10th, 

 nearly three-fifths of the growth took place. [How Crops 

 Grow, p. 205, et seq.] 



Before the seed is ripe- the lower leaves begin to turn yel- 

 low, and show that their activity is diminishing or has ceased 

 altogether, and the ripening of the plant takes place to a 

 great extent, by the removal of matters which have been pre- 

 viously stored up in the stem, leaves and roots, into the seed. 

 You may cut any of the grains at the root when the kernel 

 is in the milk, and the seed will still ripen, and although, if 

 you cut it too early, the kernel will shrink, it will be per- 

 fect in its parts and serviceable as seed-grain. 



It thus appears that the cereal plant grows from the soil 

 and atmosphere until the seed arrives at a certain stage of 

 development, then the activity of the roots and foliage de- 

 creases, the acquisition of food from external sources gradu- 



