182 PLANT GROWTH 



growth of the stems, and an extreme loss of dry weight. 

 Growth hormones may be inactivated in the Hght. The 

 radish in full sunlight carried on photosynthesis to more than 

 regain all the loss in the early germination, but the pea has 

 regained only a small amount of the earlier loss. Most plants 

 should not be transplanted until the seedlings have regained 

 a supply of stored food. 



Growth is often defined to include all the activities in the 

 development of the plant, including: cell division (see Chap- 

 ter 4) , synthesis of protoplasm, elongation of the cells, differ- 

 entiation of the cells, and the final maturing of the cells for 

 their special functions. The synthesis of the protoplasm and 

 cell walls requires food as the building material and oxygen 

 for respiration. The elongation of the cells depends on the 

 osmotic concentration and turgor pressure against the walls 

 and on the growth-regulating hormone. The cellulose cell 

 walls must stretch and the growth hormone appears to regu- 

 late the resistance to stretching. A comparison of the green 

 weights and the dry weights under these different conditions 

 of light, as well as the size and the shape as shown in the 

 photograph, illustrates how these water and growth relations 

 are manifest in seedlings when they are influenced by vary- 

 ing amounts of light. 



Available water also influences growth in size and in dry 

 weight. When water is deficient growth will be reduced 

 because of a decreased turgor pressure and a relatively 

 smaller increase in green weight than in dry weight will be 

 found. Under drought conditions the stomata may close as 

 explained in Chapter 13, causing a further decrease in the 

 rate of growth in size and in dry weight because of the 

 retarded photosynthesis. 



Since the turgor pressure is so important it is evident that 

 it will be greater at night because of the decreased transpira- 

 tion and that growth (stretching) will be greater than in the 

 day. This can readily be determined by night and morning 



