354 HOW CROPS GROW. 



Finally, E. Hoffmann (JaJiresbericlit fiber Agricultur 

 Chem., 1864, p. 110) found, in special experiments with 

 24 kinds of agricultural seeds, that light exercises no 

 appreciable influence of any kind on germination. 



The time required for Germination varies exceed- 

 ingly according to the kind of seed. It is said that the 

 fresh seeds of the willow begin to sprout within 12 hours 

 after falling to the ground. Those of clover, wheat, and 

 other grains, mostly germinate in three to ten days. 

 The fruits of the walnut, pine, and larch lie four to six 

 weeks before sprouting, while those of some species of 

 ash, beech, and maple are said not to germinate before 

 the expiration of one and a half or two years. 



The starchy and thin-skinned seeds quicken most 

 readily. The oily seeds are in general more slow, while 

 such as are situated within thick and horny or other- 

 wise resistant envelopes require the longest periods to 

 excite growth. 



The time necessary for germination depends naturally 

 upon the favorableness of other conditions. Cold and 

 drought delay the process, when they do not check it 

 altogether. Seeds that are buried deeply in the soil may 

 remain for years, preserving, but not manifesting, their 

 vitality, because they are either too dry, too cold, or 

 have not sufficient access to oxygen to set the germ in 

 action. 



Notice has already been made of the frequent presence 

 in clover-seed, for example, of a small proportion of 

 seeds that have a dense coat which prevents imbibition 

 of water and delays their germination for long periods. 

 See p. 335. 



To speak with precision, we should distinguish the 

 time from planting the dry seed to the commencement 

 of germination, which is marked by the rootlet becom- 

 ing visible, and the period that elapses until the process 

 is complete ; i. e., until the stores of the mother-seed are 



