26 ELEMENTARY BIOLOGY 



others of which may have nothing at all to do with it. In 

 order to find out just what it is that causes the sprouting, 

 we must consider the effect of each of the various factors of 

 the seed's surroundings by itself. 



41. The environment. Now, in what ways do the conditions 

 surrounding a seed in the ground differ from the conditions in 

 a box or a jar ? There may be a difference as to temperature, 

 or as to the air, or as to the amount of water, or as to the light, 

 or as to some of the chemical substances present in the soil. 

 Experiments have been made with every one of these factors, 

 and we also have a great deal of experience that will help us to 

 answer this question in part. 



Most of us know that seeds kept in jars will not sprout, 

 whether they are kept in the dark or exposed to light. It is 

 therefore safe to conclude that putting seeds in the ground 

 brings about their germination not on account of darkness, but 

 on account of some other factor. We also know that seeds 

 kept in a warm place and seeds kept in a cool place will both 

 fail to sprout, as long as they are in our jars or boxes. The 

 soil may be cooler than our storeroom, or it may be warmer ; 

 but it is not this that makes them sprout in the ground. Per- 

 haps the soil keeps some of the air away from the seed ; but 

 filling a jar with seeds and closing it up tight will not make 

 them sprout. So it cannot be the absence of air by itself, 

 nor the presence of air by itself, that causes the seeds in the 

 ground to germinate. 



If we consider the chemical substances present in the soil, 

 our usual experience tells us nothing at all. Perhaps there are 

 certain substances there that cause the sprouting. We might 

 find out by trying some of them. The chemist can tell us 

 what there is in the soil, and he can also prepare the different 

 kinds of stuff in a pure condition. But if we place the seeds 

 in boxes containing the various ingredients of the soil, such as 

 sand, clay, and various salts, we shall find that none of the 

 seeds sprout. The failure of the seeds to sprout under these 



