PLANTS 



321 



starch and proteid, which are so changed by water and 

 the digestive ferments ill the seeds that they can be used 

 as food by the growing plant 

 until it has roots capable of tak- 

 ing food in solution from the 

 ground, and leaves which take 

 the carbon dioxide and oxygen 

 from the air (Figure 282). 



Roots. In the germination of 

 many plants the roo't grows into 

 the soil first, to get water and 

 food so that the top of the plant 

 may grow. With other plants, 

 however, the top and the root 

 seem to grow simultaneously. 



There are several factors which 

 determine the direction taken by 

 roots. Gravity and water are the 

 most important of these factors 

 (Figure 283). Water is always 



found below the Surface Of the FlG - 279 - Fertilization of the 



earth, but sometimes at such 



depths that roots must go long distances to secure a supply. 

 Many plants have a greater area of root surface than of 

 branch surface. In arid regions tree roots have been 

 known to penetrate the earth to a depth of sixty feet in 

 search of water. 



FIG. 2*80. Seeds of the Milkweed Dispersed by 

 the Wind. 



