lO 



WHAT THE PLANT JS MADE OF [chap. 



fail to germinate when the usual test is made, although 

 they are perfectly formed and will eventually start into 

 growth after a lapse of time. In some of these cases 

 where the seed coat is obstinate the penetration of 

 water may be hastened by various mechanical devices ; 

 the seed coat may be cut or filed through (gardeners do 

 this with Canna seeds), or the seeds may be dipped for 

 a moment in boiling water, or even soaked for a short 

 time in strong sulphuric acid, which is then quickly 

 washed away. Certain tree seeds only germinate after 

 a forest fire has swept over the ground ; heat has been 

 necessary to break up either the nut in which the seeds 

 were enclosed or the seed coat itself The object is in 

 all cases the same, to begin the breakdown of the seed 

 coat and admit moisture to the embryo. 



Table II.— Temperatures required by Germinating Seeds, 

 (Fahrenheit.) 



Besides requiring a provision of moisture, seeds will 

 only germinate within certain well-defined limits of 

 temperature ; for each species there is a minimum 

 temperature below which no germination will take 

 place, above that minimum the rapidity of germination 

 increases with the temperature up to an optimum 

 point, beyond which it declines until an upper limit is 

 reached, which again stops germination entirely. The 

 lower limiting or minimum temperatures for germination 



