EMERGENCE OF SEEDLING 439 



(19 1 6) and Leonhardt (19 15) and the discussion by Goebel 

 (1920) should be consulted. 



Dicotyledons germinating underground in light have the 

 same difficulty in penetrating the soil as the oat. The 

 curvature of the axis tends to straighten out ; the leaves 

 and the cotyledons tend to expand and to spread apart so 

 that resistance is increased ; growth in length is depressed. 



§ 7. Mode of Growth 



In a certain sense the seedling penetrating the soil in 

 the dark may be said to grow ; it increases in size, and 

 undergoes development, yet it does not increase its substance. 

 In fact, if the dry weight of the seedling, after a few days' 

 grovi1:h in the dark, is compared with that of the seed, it is 

 found to be less ; organic material has been used up in 

 respiration and no assimilation has occurred to make good the 

 loss ; the same is true of those seedlings in which assimila- 

 tion lags behind chlorophyll formation, even when they are 

 grown in light. It is in general characteristic of the plant 

 that increase in size — extension — is due chiefly to the absorp- 

 tion of water by cells which have completed their divisions, 

 and there is a consequent great increase in volume, and often 

 change in shape, unaccompanied by any local increment in 

 organic matter. The formation of the cells by division and 

 their partial differentiation takes place in special meriste- 

 matic regions, the growing points of root and shoot, and the 

 cambium ; the region of extension lies behind, sometimes 

 well behind, the growing-point. Growth has been measured 

 by increment of length, of area, of volume, and by increase 

 in dry weight. This last measures the net result of the plant's 

 metabolic activities ; it sums up the whole of the complex 

 processes of its chemistry, of which assimilation in one 

 direction and respiration in the other are the two most 

 obvious. Though measurements of length, etc., may yield 

 valuable results for particular organs, the real growth of the 

 plant as a whole, the expression of its power to synthesise 

 and construct, is measured only by its increase in dry weight. 



