SIR J. LUBBOCK PlITTOEIOLOGICAL OBSEHYATTOXS. 3G9 



It is very impervious to water, and may be advantageous to tlic 

 embryo In resiritini; the attacks of drought and of insects, and 

 perhaps if the seed be swallowed by a bird, by protecting it 

 fi-om being digej^ted. On the other band it does not split open, 

 and is too tough to bo torn by the embryo. The cotyledons, 

 therefore, if tbey had widened, as tbey might otherwise have 

 done, would have found it impossible to emerge from the seed. 

 They evade the difficulty, however, by remaining narrow. On 

 the "other hand, in G. Aparine the pericarp is much thinner 

 (figs. 71-73), and the embryo is able to tear it open {^I'r. To). 

 In this case, therefore, the cotyledons can safely widen without 

 endauiiering their exit from the seed. 



an 



The thick corky covering of G. saccliaratum 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. saccliaratum 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 arc flat or nearly so, but a 

 larae number are enabled to widen themselves by being more or 



s 



less folded. One form of this is afforded by the Kadish {Bapha- 

 nus) and Brassica (fig. 3). Tig. 74 shows a seed of Baphanus 

 safivus, and' as shown in figs. 75-78, the latter of which rcpre- 

 ents a voung 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 Darwm*, who 

 attributed the inequality to the fact of " a store of nutriment 

 bein- 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 inequality 

 between the cotyledons. Why should it affect one more than 

 the other ? I venture to suggest that the difference is rather due 

 to the position of the embryo in the seed, which in some cases 

 favours one cotyledon more than the other. For instance, in 



* ' MoTemcnts of Plants,' p. 9A, 



