564 GROWTH, ASSIMILATION, AND ACCUMULATION 



Branch roots develop on the main root axis, as previously described in 

 Chap. XVII, from the pericycle (usually) which remains in an essentially 

 meristematic condition for some distance back of the growing point of the 

 root. Such branch roots progress through the soil by elongation of the cells 

 produced as a result of cell division in the apical meristem, and usually produce 

 branch roots in turn. 



Measures or Indices of Growth. — It is frequently desirable to give 

 some sort of a quantitative expression to the amount of growth which is 

 accomplished by a plant or a group of plants during a given period of time. 

 The principal indices which have been employed for this purpose are ( i ) in- 

 crease in the length of the stem, root, or other organ of the plant, (2) in- 

 crease in the area of the leaves, (3) increase in the diameter of the stem (or 

 other organ), (4) increase in volume (especially of fruits), (5) dry weight 

 increment, and (6) fresh weight increment. 



All of these indices have at least a limited value as measures of growth, 

 especially from various practical standpoints. Determinations of the height 

 and diameter growth of forest trees, for example, are standard forestry prac- 

 tices as indices of the productivity of forests, and have considerable practical 

 value for such purposes. Similarly the number of tons of dry hay or the 

 fresh weight of cabbage or spinach produced per acre would usually be an 

 adequate measure of growth to the mind of the practical farmer. 



Each of the indices listed above, however, measures only certain quantita- 

 tive phases of growth. A yardstick can measure only length, a balance only 

 weight, but growth phenomena generally involve not only such quantitative 

 changes as expansion in length and girth and increase in weight, but qualitative 

 aspects as well. How, for example, could the relative development of the 

 vegetative and reproductive phases of growth be expressed in terms of any 

 of the units listed above? Yet qualitative differences in growth are often of 

 as great or greater scientific significance or practical importance as quantitative 

 differences. The floriculturist is not primarily interested in the pounds of 

 plant substance produced nor the height to which his plants grow, if they 

 bear flowers which will be attractive to his customers. Likewise the orchardist 

 is much more interested in the development of the fruits on his trees, than 

 in the increase in the height or weight of their vegetative organs. Evidence 

 of this difficulty in giving adequate expression to the results of growth 

 phenomena is seen in the common expedient of investigators in relying upon 

 photographs as a means of recording the results of their experiments upon the 

 growth of plants. 



Growth Curves. — The generalized aspects of the rate of growth of 

 plants can often be given definite expression in graphical language. Such 



