CONDITIONS WHICH INFLUENCE GROWTH 245 



of the sun ; syringing with ice-cold water is often a useful method 

 of thawing them. 



In long-continued frost the water frozen on the outside of the 

 cells may gradually evaporate into the dry cold air ; under such 

 circumstances the frozen parts shrivel and die of thirst. 



Dormant seeds contain little water and are able to withstand 

 the lowest temperature attainable without injury ; recently Dewar 

 and Dyer found that the seeds of mustard, wheat, barley, pea 

 and other plants germinated freely after being soaked for six 

 hours in liquid hydrogen, the temperature of which was 453° F. 

 below freezing-point. 



In actively growing plants the protoplasm becomes disorganised 

 and its vital powers destroyed at temperatures about 45° or 50° C. 



Many dry seeds withstand a dry heat of 80° C. or higher for 

 an hour or longer; after soaking, however, they are killed by 

 10 to 30 minutes' exposure to a temperature of 51° or 52° C. 



(ii) Water. — Water is necessary for the maintenance of the 

 turgidity of the growing cells. It is itself a food-material and is 

 also essential as a vehicle for the transport of foods and food- 

 materials needed for the nutrition of the growing organs. 



When plants from the beginning of their lives suffer from want 

 of water their size is much diminished, although in other respects 

 their development appears normal; individually they become 

 dwarfs. 



On persistently dry soils and in dry seasons the bulk of the 

 hay crop, the size of the 'roots' of turnips, the height of the 

 straw of cereals, and the size of the various members of plants 

 are proportionately decreased, while in damp, seasons or upon 

 soils which hold considerable amounts of water, the growth of 

 plants is much increased. The growth and consequent size of 

 plants in pots is similarly increased or decreased by judiciously 

 varying the water-supply during the time that growth is pro- 

 ceeding. 



Somewhat sudden diminution in the supply of water results in 



