174 ELEMENTARY STUDIES IN BOTANY 



and similar appendages are developed in connection with the 

 bracts about the head of fruits of cocklebur and burdock (Fig. 

 144). Fleshy fruits are attractive as food to certain birds 

 and mammals. Many of the seeds (such as those of grapes) 

 are able to resist the attacks of the digestive fluids and es- 

 cape from the alimentary tract in a condition to germinate. 

 104. Conditions for germination. How long seeds may 

 retain their vitality is a question that cannot be answered 

 definitely, for it depends upon the kind of plant and also 

 upon the conditions in which the seeds are kept. The stories 



of the germination of 

 wheat obtained from the 

 wrappings of Egyptian 

 mummies have proved to 

 be myths, but they are 

 still in circulation. It 

 was a common observa- 

 tion that when the origi- 



FIG. 144. -Heads of fruit of cocklebur (A) and nal S d f the 

 burdock (), showing grappling appendages wag broken UD in 

 of the bracts (involucre). After KERNEK. 



ing, plants often sprang 



up that seemed new to the region. Of course such plants 

 came from seeds, and it is possible that they had been 

 kept for a time in conditions unfavorable for germination 

 until ploughing made the conditions favorable. But 

 it must be remembered that seeds are scattered over 

 wide areas every year, and that " new plants " may spring 

 up on freshly ploughed ground whose seeds have never 

 " waited " at all. 



Three conspicuous conditions for seed-germination are 

 recognized ; namely, moisture, a suitable temperature, and 

 oxygen. When seeds are " planted," the soil is used as the 

 most convenient source of continuous moisture ; but of 

 course seeds will germinate just as well on moist blotting 

 paper or on moist sand. The soil has the great incidental 



