166 PLANT PHYSIOLOGY 



kind of cell, and hence each organism, has a maximum 

 which its size normally never exceeds. Cells which have at- 

 tained their maximum size can continue to contribute to 

 the growth of an organ only after doing what will make 

 possible the formation of new protoplasm. Cell-division, a 

 process of rejuvenation, accomplishes this. The smaller 

 daughter cells, each with its own nucleus instead of with 

 only a share of the single nucleus of the undivided cell, are 

 more vigorous than the mother-cell at maturity ; they form 

 new protoplasm as well as other products from the food 

 furnished them ; and they then increase in size. In a meris- 

 tematic tissue, such as that at the tip of the stem or root 

 or in the cambium, we have the first and the fundamental 

 stage in the process of growth, namely, the formation of new 

 protoplasm and of new cells. Behind the tip of stem and 

 root, and on either side of the cambium, we have the later, 

 the evident stage, when increase in size or volume, in mass 

 and in substance, takes place. 



Evident growth takes place when the body or any part of 

 it permanently increases in volume or in size. Increase in 

 substance, which results in increase in weight, may take 

 place after all growth has ceased. It may be merely the 

 storing up of food elaborated at one time to be used later, 

 or it may be the absorption of water. This last, however, 

 invariably results in the increase in volume of the parts, 

 or in the increase in turgor, or both at once. Any ab- 

 sorption of water by dry animal and vegetable substances 

 is followed immediately by swelling a phenomenon from 

 which hypotheses regarding the minute structure of organ- 

 ized bodies have been more or less successfully deduced. 

 Beyond the point at which swelling by the intussusception 

 of molecules of water between the molecules or groups of 

 molecules of the formed substances would cease, increase in 

 volume may still go, by reason of the structure and compo- 

 sition of cells. The presence of osmotically active substances 

 in cells which can absorb water ensures increase in pressure 

 within the cells, and this pressure will distend the enclosing- 

 protoplasmic and cellulose or other walls. Up to a certain 

 point, easily conceived but not easily determined, the in- 



