CHAP. VIII. 
SEED. 
303 
has a coleorhiza or rootsheath, this is soon perforated by the 
radicle contained within 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 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 radicle 
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 
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 ; 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 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 vacuo, 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 
