166 FIELD CROPS FOR THE COTTON-BELT 



the formation of new cells and the enlargement or extension 

 of old cells. The former process occurs in what is known 

 as the formative region where the cells are constantly 

 dividing. Adjacent to this is the elongating region where 

 the cells expand or enlarge by absorbing large quantities 

 of water. Both of these changes bring about a rapid in- 

 crease in the length and size of the plant. During the first 

 three weeks of growth all the organs of a corn plant are 

 formed such as the full number of leaves and nodes, the 

 embryonic tassel, ears and tillers, and most of the main 

 . roots. Subsequent growth involves largely the extension 

 of these parts together with certain changes of internal 

 structure characterized by the deposition of starch in the 

 ear and the strengthening of the fibrous tissues. From 

 the standpoint of the farmer the practical measure of 

 growth is the yield of the crop. 



208. The factors of growth. — Growth is conditioned 

 upon vitahty or the life of the plant, and heredity or the 

 force which operates to reproduce specific forms. These 

 are the internal factors of growth. The external factors 

 are moisture, a suitable temperature, oxygen, the various 

 nutrients and food materials, and light. 



209. The growth of roots. — Under favorable condi- 

 tions the roots of corn may elongate at the rate of more 

 than an inch a day. The formation of new cells occurs 

 in the region just behind the root-cap known as the apical 

 meristem. Extending back from this zone of cell division 

 for a very short space is the zone of elongation in which 

 the newly formed cells increase rapidly in size. This 

 rapid formation and elongation of cells tends to push 

 forward the root-cap and the root is thus gradually ex- 

 tended in the soil. New lateral roots develop in the region 

 immediately behind the growing root-tip. Under favor- 



