1890.] on the Shapes of Leaves and Cotyledons. 109 



becomes larger. In many other Crucifers, though the cotyledons are 

 not folded, they are what is called " incumbent " — that is to say, they 

 are folded on the radicle, and the outer one has therefore more room 

 than the other. 



Unsymmetrical Cotyledons, 



In other cases, as in the Geraniums, Laburnum, Lupines, &c., there 

 is inequality, not between the two cotyledons, but between the two 

 halves of each cotyledon. In the geraniums this is due to the manner 

 in which the cotyledons are folded. In the cabbage and mustard 

 we have seen that one cotyledon is folded inside the other ; in the 

 geranium they are convolute, one half of each being folded inside 

 one half of the other, the two inner halves being the smaller, the two 

 outer the larger ones. 



In the laburnum, where the arrangement is very similar, the 

 inequality in the two sides of the cotyledon is due to the inequality 

 between the two sides of the seed. 



Subterranean Cotyledons. 



I have already observed that in some cases the cotyledons occupy 

 the whole of the seed, which in more or less spherical seeds is 

 effected, either by a process of folding and packing, or by the 

 cotyledons becomiiag themselves more or less thickened, as in peas 

 and beans, nuts and chestnuts. This is the reason why such seeds 

 fall more or less ^readily into two halves, the radicle or plumule 

 being so small in comparison as generally to escape notice, though, 

 if a horse-chestnut is peeled, the radicle appears as a sort of tail. 



In certain beans the cotyledons sometimes emerge from the seed, 

 sometimes remain underground. In others, as also in the oak and 

 horse-chestnut, they never leave the seed, or come above ground : 

 they have lost the function of leaves, and become mere receptacles 

 of nourishment. 



Did it ever occur to you to think, when you have been eating 

 walnuts, why their structure is so complex, and why the edible part 

 is thrown into those complicated lobes and folds? The history is 

 very interesting. 



In the Walnut the cotyledons now never leave the seed, but in an 

 allied genus, Pterocarya, they come above ground as usual, and are 

 very peculiar in form, being deeply four-lobed. The reason of this 

 is very curious. The fruit is originally much larger than the seed, 

 but, as it approaches maturity, the hard woody tissue disintegrates 

 at four places, leaving thus four hollow spaces. Into these spaces 

 the seed sends four projections, and into these four projections each 

 cotyledon sends a lobe. Hence the four lobes. 



Now in the walnut a very similar process takes place, only the 

 hollow spaces are much larger, so that, instead of a solid wall, with 



