Ways in Which Plants Increase in Size 345 



attained by increasing the size of cells beyond the normal, not by 

 increasing their number. 



But while enlargement and development are separate in their 

 nature, and commonly occur apart from one another, neverthe- 

 less they are often intermingled more or less. The very act of 

 development, indeed, entails some increase of size, and enlarge- 

 ment is attended by some cell divisions in connection with adjust- 

 ment of parts; and no doubt, furthermore, there are structures 

 which develop and enlarge simultaneously. 



As growth comes near to completion, and sometimes much 

 earlier, the cells undergo such further changes as fit them more 

 perfectly for particular functions. Such changes are designated 

 maturation. Walls thicken in places and are absorbed in others; 

 they develop spirals, rings, or other thickenings, and hollow pits 

 or other depressions; while various changes take place as well in 

 the contents, which often are transformed to secretions of very 

 specialized function. Moreover, as cells increase in perfection of 

 adaptation to their functions, they lose at the same time their 

 power of division, so that when fully mature they are incapable 

 of further development or reproduction. But these changes in 

 the main have already been considered in the chapters on Pro- 

 toplasm and Metabolism. 



Before leaving this aspect of growth, we should summarize for 

 completeness the other physical and chemical phenomena thereof, 

 most of which have been considered in various connections earlier 

 in this book. Thus, there is always a large conversion of stored 

 food into new walls, protoplasm, and sap substances, resulting in 

 the collapse of the storage parts of sprouting structures, like 

 potatoes, bulbs and seeds. Again, respiration, the releaser of 

 energy, is indispensable to growth, which demands much of it; 

 and so close is the connection of the two, that whatsoever stops 

 the one stops also the other. Therefore, oxygen being essential to 

 respiration, if the oxygen supply be cut off from the growing 

 plant, as happens often in nature through flooding with water, 



