770 PHYSIOLOGY. 



are gathered. If preserved till they are quite dry, in some 

 cases the process of germination is months in being effected. 

 The seeds of the garden cresses will frequently germinate in 

 twenty-four hours, but the majority of seeds do not germinate 

 for from six to twenty days, and some require months or even 

 years. Germination is generally prolonged when the embryo 

 is invested by hardened integuments or albumen ; and it is 

 usually of a limited duration in exalbuminous seeds, more 

 especially if such seeds have thin soft integuments. Heat is the 

 agent wliich most accelerates germination. 



Conditions requisite for Germination. — A certain amount of 

 heat and moisture, and a free communication with atmospheric 

 air, are in all cases necessary to the process of germination. 

 Electricity is also considered by some observers to promote it, 

 but its influence in the process is by no means proved, and if 

 exerted it is apparently of but little importance. Absence from 

 light, however, is another condition, which is favourable to 

 germination in most cases. 



Moisture is required to soften the parts of the seed and to 

 take up all soluble matters ; the cells of which seeds are com- 

 posed are in this way enabled to expand, and the embryo to 

 burst through the integuments. 



Heat is necessary to excite the dormant vitality of the embryo, 

 but the amount required varies very much in different seeds. 

 As a general rule from 50° to 80° of Fahr. may be regarded as 

 most favourable to germination in temperate climates, but 

 some seeds will germinate at a temperature of 35° Fahr. ; 

 and those of many tropical plants require a temperature of 

 from 90° to 120° Fahr., or sometimes much higher, for germi- 

 nation. 



Air, or at least oxygen gas, is required to combine with 

 the superfluous carbon of the seed, which is thus evolved with a 

 sensible increase of temperature, as is well seen in the malting 

 of Barley, as carbonic acid. The necessity of a proper supply 

 of oxygen is proved by the fact, that seeds will not germinate 

 when buried too deeply in the soil, or when the soil is impervious 

 to air. This explains why seeds frequently lie dormant at 

 great depths in the soil, and only germinate when that soil 

 is brought to the svirface ; and hence we see the necessity of 

 admitting air to seeds by the ordinary operations of agri- 

 culture. 



Process of Germination. — When the above requisites arc sup- 

 plied in proper proportions to suit the requirements of different 

 seeds, germination takes place, but should either be wanting or 

 in too great amount, the process is more or less impeded, or 

 altogether arrested. The most favourable seasons for germina- 

 tion are spring and summer ; and seeds sprout most readily in 

 loose pulverised and properly drained soil, at a moderate depth, 



