40 



PLANT BIOLOGY 



becomes incorporated in the plant tissue, so that these 

 crops are high in their nitrogen content. Inasmuch as 



nitrogen in any form is 

 expensive to purchase in 

 fertilizers, the use of legu- 

 minous crops to plow under 

 is a very important agricul- 

 tural practice in preparing 

 the land for other crops. 

 In order that leguminous 

 crops may acquire atmos- 

 pheric nitrogen more freely 

 and thereby thrive better, 

 the land is sometimes sown 

 or inoculated with the nod- 

 ule-forming bacteria. 



Fig. 42.— Nodules on Vetch. 



Roots require moisture in order to serve the plant. The 

 soil water that is valu- 

 able to the plant is not 

 the free water, but the 

 thin film of moisture 

 which adheres to each 

 little particle of soil. The 

 finer the soil, the greater 

 the number of particles, 

 and therefore the greater 

 is the quantity of film 

 moisture that it can hold. 

 This moisture surround- 

 ing the grains may not 

 be perceptible, yet the 

 plant can use it. Root absorption may continue in a soil 

 which seems to be dust dry. Soils that are very hard and 



Fig. 43. — Two Kinds of Sou. that have 

 been Wet and then Dried. The 

 loamy soil above remains loose and capa- 

 ble of growing plants ; the clay soil below 

 has baked and cracked. 



