RELATION OF DEPTH OF BURIAL TO VITALITY. 17 



sprouts at the time the seeds were taken up. The number of fresh 

 sprouts in each case is indicated in a footnote to Table III. 



Unlike Table II, Table III includes names of but very few of our 

 cultivated plants. The majority belong to that class of plants com- 

 monly known as weeds. These results show that but a limited num- 

 ber of our cultivated plants produce seeds which can retain their 

 vitality for any length of time when buried in soil. On the other 

 hand, the seeds of the plants which are commonly known as weeds are 

 of strong vitality, and many of them deteriorated but very little with 

 the treatment given. This, of course, is what we would expect. By 

 natural selection the wild plants which survive are just those from 

 seeds which are capable of living in the soil for a period of time more 

 or less extended, and ultimately this factor becomes hereditary. With 

 most of the cultivated plants the seeds are gathered and carefully 

 saved from year to year, resulting in the loss of these inherited 

 characteristics. 



The mere fact that certain seeds retain their power of germination 

 for a period of years when buried in the soil brands the plants which 

 the}' produce as weeds. The length of time that such seeds can remain 

 in the soil and still retain their power of germination largely deter- 

 mines their noxiousness. In other words, it may well be said that the 

 pernicious character of weeds is directly proportional to the length of 

 time the seeds will remain viable when buried in the soil. For this 

 reason bad weeds are difficult to eradicate once the seeds are allowed 

 to mature. (See PI. II.) 



RELATION OF DEPTH OF BURIAL TO VITALITY. 



Table III shows that many of the seeds were better preserved the 

 deeper they were buried. This is probably best explained by the 

 difference in the three factors which govern germination, viz, heat, 

 moisture, and oxygen. At the greatest depth the amount of moisture 

 is always more uniform, the supply of air is greatly lowered, and the 

 temperature is much reduced. The temperature decreases very rapidly 

 as we go below the surface of the soil, and at 3| feet is comparatively 

 uniform throughout the year. Experiments conducted at McGill 

 College, Montreal, Canada, by C. H. McLeod show that at a depth of 

 40 inches below the surface of the soil the minimum and maximum 

 temperatures through the year were approximately 35° and 60° F., 

 respectively.* 



The greater number of seeds germinate best when subjected to daily 

 alternations in temperature. These alternations do not take place at a 

 depth of 3 feet below the surface; consequently there is a better 



"Trans. Roy. 8oc., Canada, Ser. 2, Vol. 7, Sec. Ill, pp. 13-16, 1901. 



