74 GERMINATION. 



oxygen, nitrogen, hydrogen, or carbonic acid, are de- 

 trimental. 



22. Supposing a seed is prevented from being exposed 

 to any of the conditions we have now mentioned, it may 

 be preserved in a state of slumbering vitality for an im- 

 mense length of time, and this seems to depend upon 

 the long power of endurance of the starchy matter it 

 contains ; but on the other hand many seeds will re- 

 main as seeds under the most peculiar circumstances, 

 even embracing some of the conditions necessary for 

 germination to take place. In the transactions of the 

 Linnsean Society of Bordeaux, is related the opening 

 of some Roman tombs which contained in their coffins 

 seeds of Heliotropium Europae, etc., and which still re- 

 tained their powers of germination ; and Passalacca 

 brought from the ruins of Thebes some grains which, 

 when examined by Fontenelle, were found to contain 

 their proper proportion of starch, but no gluten. 



23. The seed, however, being favourable to the pro- 

 cess of germination, and placed under its proper condi- 

 tions, and the changes we have spoken of having more 

 or less ensued, the expulsion of the radicle of the em- 

 bryo beyond or out of the cotyledons is seen to take 

 place ; in the case of Monocotyledonous plants the ra- 

 dicle of the embryo in bursting through its coats be- 

 comes enclosed within a sheath, whilst in Dicotyledo- 

 nous plants it is free and uncovered, the point of its exit 

 being generally at the hilum ; the cotyledons either re- 

 main below ground whilst the plumvla ascends, or they 

 rise above it and appear as leafy expansions ; in Mo- 

 nocotyledonous plants the cotyledonary body remains 

 within its envelopes. 



24. Upon the exit of the radicle it at once directs 

 itself downwards beneath the earth, and sooner or later, 

 whatever obstacles may be thrown in the way of its so 

 doing, it will effect its object. Till very lately this di- 

 rection of the radicle was supposed impossible to be 

 overcome ; but it is stated to have been effected by 

 placing mirrors over the seeds during germination, so 



