364 
PHYSIOLOGY. 
BOOK II. 
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 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 ex- 
periment 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 
seeds buried in the earth, and from other circumstances, that 
the principal conditions are, I . uniform temperature ; 2. mo- 
derate dryness ; and 3. exclusion of light : and it will be 
found, that the success with which seeds are transported from 
foreign countries, in a living state, is in proportion to the care 
and skill with which these conditions are preserved. For 
example, seeds brought from India, round the Cape of Good 
Hope, rarely vegetate freely : in this case, the double expo- 
sure to the heat of the equator, and the subsequent arrival of 
the seeds in cold latitudes, are probably the causes of their 
