30 ON SEEDLINGS 



The thick corky covering of G. saccharatum is doubtless 

 much more impervious to water than the comparatively thin 

 testa of G. Aparine. The latter species is a native of our 

 own isles, while G. saccharatum inhabits Algiers,' the hotter 

 parts of France, &c. May not, then, perhaps the thick corky 

 envelope be adapted to enable it to withstand the heat and 

 drought ? 



In all these species the cotyledons are flat or nearly so, 

 but a large number are enabled to widen themselves by being 

 more or less folded. One form of this is afforded by the 

 Eadish (Raphanus) and Brassica (fig. 3). Fig. 62, A, shows a 

 seed of Eaphanus sativus, and, as shown in fig. 62, B-E, the 

 latter of which represents a young seedling, the cotyledons are 

 applied to one another face to face, and then folded along the 

 middle. 



UNEQUAL COTYLEDONS. 



I now turn to those species in which the two cotyledons 

 are unequal in size. 



Several of these cases have been discussed by Darwin, 1 

 who attributed the inequality to the fact of ' a store of nutri- 

 ment being laid up in some other part, as in the hypocotyl, or 

 one of the two cotyledons, or one of the secondary radicles.' 

 I differ with the greatest hesitation from so high an authority ; 

 but do not see the connection between the store of food being 

 partly laid up in some other part of the plant and the in- 

 equality between the cotyledons. Why should it affect one 

 more than the other ? I venture to suggest that the difference 

 is due rather to the position of the embryo in the seed, which 

 in some cases favours one cotyledon more than the other. 

 For instance, in many cases the cotyledons are what is called 

 ' incumbent,' that is to say the radicle is folded upon one of 

 the cotyledons, and in such species the outer cotyledon is often 

 rather larger than the other, as for instance in Hesperis 

 matronalis. 



The Hemp (Cannabis) and Caylusea present us with cases 

 more or less resembling that of Hesperis. 



1 Movements of Plants, p. 94. 



