492 Minnesota Plant Life, 



ment. For the sake of clearness at least three types of growth 

 should be distinguished: i, growth of living substance; 2, 

 growth of cells, and 3, growth of tissues and organs. The first 

 takes place as a result of constructive assimilative processes, 

 and may or may not be accompanied by a visible increase in 

 size. The second may go on without increase in mass either 

 of the living substance or of the organ to which the growing 

 cell belongs. The third may occur while mass of living sub- 

 stance and cells are increasing, but sometimes even while mass 

 is stationary or diminishing. Thus by change in shape of its 

 component cells an organ may elongate, though while it does 

 this there may be no actual increase in substance. Neverthe- 

 less such an elongation might appear as visible growth. Such 

 an analysis is somewhat disregardful of refined and technical 

 terminology, but is not seriously at variance with the truth. 



Growth of living substance. The most important fact to be 

 kept in mind concerning the growth of living substance, is that 

 it is not simply an accretion of new material. Growth goes 

 on in varying degree among the innumerable component parts 

 of the living substance, each growing in its own way and at 

 its own rate. Thus results the growth in aggregate of the 

 whole. To illustrate this, let there be imagined a village full 

 of children. After a year or two had passed the children 

 would have grown and the total human weight of the village 

 would have increased. Some, however, of the children would 

 have grown more than others, some, even, might not have 

 grown at all. The final result, however, a general increase in 

 weight, was not obtained by mere accretion. In such fashion 

 does a microscopic piece of living substance grow. It grows 

 as an organism, not as an office-building nor as a snow-drift. 



For living substance, the limits of growth above which me- 

 chanical support, such as partition walls, strengthening beams, 

 and retaining membranes, becomes desirable, are generally be- 

 low the limits of unaided vision. For this reason most plant 

 cells are of microscopic size. The cell may, in a complex 

 tissue or organ composed perhaps of millions, be regarded as 

 resulting from the necessity of individualizing small pieces of 

 living substance. It is true, cells vary greatly in size. The 

 smallest are those of the bacteria. The largest are those of 



