CH. VIII, 4] GERMINATION OF SEEDS 381 



products of the slow metabolism within the tightly-sealed 

 seed coats poisons the embryo ; arid it may be that the slow 

 coagulation of the proteins destroys the essential constitu- 

 tion of the protoplasm. Between these possibilities, and 

 perhaps others, the future will decide. 



4. THE GERMINATION OF SEEDS 



The seed, its resting period completed, germinates on 

 access of water, air, and warmth. The water it needs to 

 expand its parts; the air is necessary for its respiration, 

 which is very active in all growth; the warmth is required 

 to accelerate the many physical and chemical processes in- 

 volved. As to light, that has no influence, direct or indirect, 

 in most cases, though special seeds are known which will not 

 germinate in light, and others which will not germinate with- 

 out it, doubtless for reasons incidental to some peculiarity 

 of their metabolism. 



In germination we can distinguish some seven stages. 

 First, most seeds, though not all, swell greatly tihrm,iffh^iit r 

 often to more than double their dry size, by absorption 

 of water, which enters partly by imbibition and partly by 

 osmosis. As these words imply, the absorption is forcible, 

 and thus, seeds can lift considerable weights in the ground or 

 break strong containers under experiment. 



Second, the seed coats are broken^ no matter how thick 

 lmbT~strong, by the pressure from within. In some the 

 rupture is irregular ; in others, it follows definite lines cor- 

 responding with angles or depressions of the coats. Some 

 very striking special arrangements to this end are known 

 (Fig. 273). 



Third, the digestion of the food substances begins. The 

 insoluble starches, oils, and proteins are converted by en- 

 zymes into soluble sugars, fatty acids, and peptones, as 

 manifest to the eye in the change from opacity to trans- 

 lucency, and a softening of the seed. Then the digested food, 

 absorbed by the cotyledons in albuminous seeds, though 



