SEED. 105 



botany, and constitutes one of the most iiuportunt suhjocts of 

 the slu.ient's attention. 



1*24. We have above remarked that the ba<e ot ihc xcd was 

 its point of attachment to tiie pkieenta, or that the hihun was 

 always the base., One would naturally suppose that the op- 

 posite point would be the apex ; but this is not «»;enerally the 

 case, iu ortjjotropous seeds, only, does the re;il apex corres- 

 pond with the geometrical apex. In cam|)ulitroi)ons seeds the 

 apex of the seed is nearly in contact with the base, the axis 

 of the ovule having been doubled on itself, thus bringing the 

 real apex in close proximity to the base. It is very common 

 for tho surfaces of seetis, particularly of minute ones, to be 

 marked by lines of various kinds, sometimes barely lines of 

 different color, at others by ridges or depressions, and in others 

 by dots, and in some by regularly arranged tubercles. In all 

 these cases the point of their divergence is the hilum or base, 

 and the point of their convergence the apex, S(j that by this 

 means Ihe base and apex of the seed is often determined, w Irih 

 it would be difficult to do it by any other. Having determi- 

 ned the loramen, base, and chalaza of a seed, which may gen- 

 erally be done, by simple inspection, the position of the in- 

 terior parts is with certainty known. It has been demonstiu- 

 ted that in nearly every case the radicle points towards the 

 foramen, and the plumula tow ards the chalaza. In ortholro- 

 pons seeds, therefore, the etnbryo is inverted, that is tiie radi- 

 cle points upwards ; in campulitropoiis the embryo is curved ; 

 in anatropous the embryo is erect, and so in all cases, llie 

 niycropyle being taken for the ladicie and the chalaza w hen 

 present, and the hi!um when the clialazi is not found tor the 

 plumula, the true position of the embryo is always determined. 



125. The embryo answers most important purposes in 

 classitication, since the whole vegetable kinsdom has been 

 divided into three creat classes founded on the varieties of 

 structure of the embryo. There have been found three va- 

 rieties of embryo, MonocotylcdonouSj Dicotyledonous, and 

 Acotyledonous, and it has been observed that tlic vegetables 

 arising from these ditTerently constituted em!)ryos are dis- 

 linet and peculiar in thoir constitution and mode of growth. 



12t>, Fig. 142 exhibits the dicotyledonous embryo, the ditTer- 

 ent parts of which have been already pointed out. The growth 

 of this kind of embryo produces our forest trees, and all vegeta- 

 bles having a distinct bark and pith. But, as above rem.irk- 

 C<1, it has lx?en discovered that there are vegetables with the 

 above characteristics, whose embryos are unlike the one above 

 described, some having numerous cotyledons, others but one, 



