556 SUMMARY AND CHAF. XII. 



the upper part of the hypocotyls of at least some 

 plants. 



As the arch grows upwards the cotyledons are 

 dragged out of the ground. The seed-coats are either 

 left behind buried, or are retained for a time still 

 enclosing the cotyledons. These are afterwards cast 

 off merely by the swelling of the cotyledons. But 

 with most of the Cucurbitaceae there is a curious 

 special contrivance for bursting the seed-coats whilst 

 beneath the ground, namely, a peg at the base of the 

 hypocotyl, projecting at right angles, which holds down 

 the lower half of the seed-coats, whilst the growth 

 of the arched part of the hypocotyl lifts up the upper 

 half, and thus splits them in twain. A somewhat 

 analogous structure occurs in Mimosa pudica and some 

 other plants. Before the cotyledons are fully ex- 

 panded 'and have diverged, the hypocotyl generally 

 straightens itself by increased growth along the con- 

 cave side, thus reversing the process which caused 

 the arching. Ultimately not a trace of the former 

 curvature is left, except in the case of the leaf-like 

 cotyledons ^of the onion. 



The cotyledons can now assume the function of 

 leaves, and decompose carbonic acid ; they also yield 

 up to other parts of the plant the nutriment which 

 they often contain. When they contain a large stock 

 of nutriment they generally remain buried beneath 

 the ground, owing to the small development of the 

 hypocotyl ; and thus they have a better chance of 

 escaping destruction by animals. From unknown 

 causes, nutriment is sometimes stored in the hypocotyl 

 or in the radicle, and then one of the cotyledons or 

 both become rudimentary, of which several instances 

 have been given. It is probable that the extraordi- 

 nary manner of germination of Megarrhiza Californica y 



