216 COLLEGE BOTANY 



changes will go on in stored fruits and vegetables. Cold storage 

 is used to hold these activities in check. 



Location and Rate of Growth. We have already learned 

 that growth is localized in the plant body. The most active cell 

 division is in the meristematic system, which in our higher 

 plants means the primordial nieristem at the tip of the stem 

 and the cambium of the stems and roots (see Chapter III). 

 The enlargement of the cells, the thickening of the cell walls and 

 other internal evidences of growth have also been discussed. 



The rate of growth varies with the species of plant, the part 

 of the plant, the season, the time of the day and the environ- 

 mental factors. 



( 1 ) It is very generally recognized that some species of plants 

 grow very rapidly, while others grow very slowly : the hardwood 

 trees grow very slowly, only a few inches in a season, while many 

 of our agricultural crop plants attain their full growth in a 

 very short season. Many weeds grow faster than the crop 

 plants, and it is necessary to cultivate the crop as a means of 

 protection against them. 



(2) The younger parts of many plants elongate very rap- 

 idly. We have already called attention to the zone of elonga- 

 tion in the roots (page 67). The tips of the stems also elongate 

 very rapidly, in some cases one or two or three or more inches in 

 twenty-four hours. The elongations can be very readily meas- 

 ured by special instruments (auxograph and auxanometer) . 



(3) In the temperate zones the spring and summer are very 

 generally recognized as the seasons of growth, and the fall and 

 winter as the seasons of rest. It is also very generally recognized 

 that some plants make the major part of their growth during a 

 very short period and that the remainder of the season is utilized 



