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POPULAR SCIENCE REVIEW. 
the soil, the hypocotyl, as the axis below the cotyledons is 
called, begins to develope, at least in those seeds which elevate 
their cotyledons above ground ; but it is the plumule or 
epicotyl which alone does so of seeds which retain their 
cotyledons below the soil. In either case the organ com- 
mences its growth in the form of an arch, such being the 
result of an innate tendency in the case of the plumule to 
assume that form ; the inner concave surface then growing 
more rapidly than the outer and convex, tends to raise the 
summit of the arch out of the ground. The apex of the stem, 
being ultimately freed from the soil, soon straightens itself and 
becomes erect. 
In whatever position the seeds may lie, apogeotropism acts 
upon the arch, and the two legs become vertical; the entire 
arch circumnutates more or less the whole time as the nature 
of the soil permits. Mr. Darwin aptly compares the whole 
process of extrication of the arch from beneath the soil to a 
man thrown down on his hands and knees, at the same time to 
one side, by a load of hay falling upon him. He would first 
endeavour to get his arched back upright, wriggling at the 
same time in all directions to free himself a little from the 
surrounding pressure ; and this may represent the combined 
effects of apogeotropism and circumnutation. The man, still 
wriggling, would then raise his arched back as high as he 
could ; as soon as the man felt himself at all free, he would 
raise the upper part of his body, whilst still on his knees and 
still wriggling. This will represent the bowing backwards of 
the basal leg of the arch, which aids in the withdrawal of the 
cotyledons from the buried and ruptured seed-coats. 
Cotyledons. — Colytedons are in constant movement, chiefly 
in a vertical plane, and commonly rise up once and descend 
once in 24 hours ; some move much oftener, thus, those of 
Ipomcea coerulea moved thirteen times either upwards or down- 
wards in 16 h 18 m . As the motion when perfected gave ellipses, 
cotyledons maybe said to circumnutate. In a large majority of 
instances observed by Mr. Darwin, the cotyledons sank a little 
downwards in the forenoon, and rose a little in the afternoon 
or evening, thus exhibiting a certain periodicity in their 
movements, no doubt in connexion with the daily alternations 
of light and darkness. When the cotyledons rise or fall to 
such a degree as to be vertical, or at an angle of at least 60° 
above or below the horizon, they are said to be asleep ; the 
object gained being, as in leaves, to escape injury by radiation 
at night. 
Stems. — Several experiments with stems of plants of various 
orders, showed that they continually circumnutate ; and in the 
case of stem-climbers the circumnutation is of the most perfect 
