FUNCTION.] PHENOMENA OF (IHILMJ NATION. 265 



species : sometimes it dilates equally in all directions, and 

 bursts through its coat, which thus becomes ruptured in every 

 direction ; more frequently the radicle passes out at the hilum, 

 or near it, or at a point apparently provided by nature for 

 that purpose, as in Caniia, Commelina, &c. If the radicle 

 has a coleorhiza or root-sheath, this is soon perforated by 

 the radicle contained within it, which passes through the 

 extremity ; as in Grasses, and most monocotyledonous plants. 

 The cotyledons either remain under ground, sending up their 

 plumule from the centre, as in the Oak ; or from the side of 

 their elongated neck, as in Monocotyledons ; or they rise 

 above the ground, acquire a green colour and perform the 

 ordinary functions of leaves, as in the Radish and most plants. 

 In the Mangrove, germination takes place in the pericarp, 

 before the seed falls from the tree ; a long thread-like caulicle 

 is emitted, which elongates till it reaches the soft mud in which 

 such trees usually grow, where it speedily strikes root, and 

 separates from its parent. Trapa natans has two very unequal 

 cotyledons : of these, the larger sends out a very long petiole, 

 to the extremity of which are attached the radicle, the plu- 

 mule, and the smaller cotyledon (Mirbel). Cyclamen germi- 

 nates like a Monocotyledon : its single cotyledon does not 

 quit the seed till the end of germination ; and its caulicle 

 thickens into a fleshy knob, which roots from its base. The 

 Cuscuta, which has no cotyledons, strikes root downwards, 

 and lengthens upwards, clinging to any thing near it, and 

 performing all the functions of a plant, without either leaves 

 or green colour. In Monocotyledons, the cotyledon always 

 remains within the seminal integuments, while its base 

 lengthens and emits a plumule. In Cycas, which has two 

 cotyledons, the seminal integuments open, and the radicle 

 escapes. 



It has already been seen, that, under certain circum- 

 stances, the vitality of seeds may be preserved for a very 

 considerable length of time; but it is difficult to say what 

 are the exact conditions under which this is effected. We 

 learn from experiment that seeds will not germinate if 

 placed in vacuo, or in an atmosphere of hydrogen, nitrogen, 

 or carbonic acid; but no such conditions exist in nature, 



