[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 391 



The disappearance of fats and proteins through digestion is not difficult 

 to observe, but methods of detecting the intermediate and end products 

 are somewhat compHcated (Fig. 173). 



6 9 



TIME IN DAYS 



Fig. 173. Changes in amounts of foods in the cotyledons of germinating sun- 

 flower seeds. The sugar moves out of the cotyledons into the hypocotyl about as 

 rapidly as it is formed from the disappearing fats and proteins; some of it is tem- 

 porarily condensed to starch. Data from E. C. Miller. 



A part of the sugar is oxidized in respiration, and during germination 

 the temperature of seeds is raised slightly by the heat energy liberated. 

 Another part of the energy is used in the chemical transformations in- 

 volved in the synthesis of pectic compounds, the constituents of proto- 

 plasm and certain other parts of the cell. As a result of respiration, seeds 

 and seedlings progressively decrease in weight if germination takes place 

 in the dark. 



Delayed germination. The seeds of most wild plants do not germinate 

 immediately after they fall from the plant, even when placed in condi- 

 tions ordinarily most suitable for rapid growth. Even under experi- 

 mental conditions the seeds of many plants germinate only after the lapse 

 of several weeks or months, or even years. The length of the delay in 

 germination of seeds of different plants is often a specific characteristic. 

 The seeds of hundreds of species, both wild and cultivated, have been 

 tested and the conditions most favorable for their germination have been 

 discovered. Gennination of these seeds does not occur previous to cer- 

 tain changes in the composition and structure of the embryos or of seed 



