CHAPTER X 

 GROWTH 



85. General Laws of Growth and Their Mathematical Formu- 

 lation. — In considering the growth of plants it is customary, 

 first of all, to emphasize the increase in size of growing organs, 

 growth in height often being distinguished from growth in thick- 

 ness. Increase in size, however, is not always connected with an 

 increase in the amount of organic substances. It has been empha- 

 sized that seeds growing in darkness may actually lose in total dry 

 weight. 



A general increase in size of plants is, therefore, connected not 

 so much with the accumulation, as with a redistribution of organic 

 substances. During growth, the reserve materials are transformed 

 into soluble forms which go into the building of the plastic substances 

 constituting living protoplasm. Accretion in the amount of proto- 

 plasm results in an increase of the number of cells and the general 

 dimensions of a plant. It must be noted, though, that not every 

 increase in size should be termed growth. The swelling of seeds, 

 for instance, is not growth. By growth must be understood only 

 such an increase in size that is permanently acquired through 

 internal processes. 



An exact study of the phenomenon of growth requires, in the 

 first place, a precise measurement of it. Several methods are avail- 

 able for this purpose. The simplest of them consists in measuring 

 the length of the growing organs or of whole plants at definite 

 time intervals by means of a properly subdivided ruler. As 

 the growth of plants on the whole is rather slow, this compara- 

 tively crude method may be used only when measurements are 

 taken not oftener than every 24 hr. When it is desired to follow 

 the growth of a plant at shorter intervals of time, a horizontal 

 microscope will be found convenient. It is focused upon the end 

 of the growing organ, as for instance, the tip of a blade of grass or 

 the end of a root, and then by means of an ocular micrometer the 



279 



