CHAP. Vlil. SEED. 303 



has a coleorhiza or rootsheath, this is soon perforated by the 

 radicle contained witliin it, which passes through the ex- 

 tremity; 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 grovmd, 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 radicle 

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

 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 

 germinates like a monocotyledon : its single cotyledon does 

 not quit the seed till the end of germination ; and its radicle 

 thickens into a fleshy knob, which roots from its base (Mirbel). 

 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 ; wliile 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 circumstances 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 experi- 

 ment that seeds will not germinate if placed in vaciw^ or in an 

 atmosphere of hydrogen, nitrogen, or carbonic acid; but no 

 such conditions exist in nature, and, therefore, it cannot be 

 they which have occasionally preserved vegetable vitality in the 

 embryo plant for many years. Perhaps the following remarks, 

 in a work lately published by the Society for the Diffusion of 

 Useful Knowledge, may throw some light upon the subject : — 



" It may, upon the whole, be inferred from the duration of 



