96 SEED. 



each other, is of great practical importance in descriptive botany, 

 and constitutes one of the most important subjects of the stu- 

 dent's attention. 



168. We have above remarked that the base of the seed was 

 its point of attachment to the placenta, or that the hilum was 

 always the base. One would naturally suppose that the oppo- 

 site point would be the apex ; but this is not generally the case. 

 In orthotropous seeds, only, does the real apex correspond with 

 the geometrical apex. In campylotropous 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 the surfaces 

 of seeds, 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 con- 

 vergence the apex, so that by this means the base and apex of 

 the seed is often determined, when it would be difficult to do it 

 by any other. Having determined the foramen, base, and 

 chalaza of a seed, which may generally be done by simple inspec- 

 tion, the position of the interior parts is, with certainty, known. 

 It lias been demonstrated, that in nearly every case the radicle 

 points toward the foramen, and the plumula toward the chalaza. 

 In orthotropous seeds, therefore, the embryo is inverted, that is, 

 the radical points upward ; in campylotropous the embryo is 

 curved ; in anatropous the embryo is erect; and so in all cases, 

 the mycropyle being taken for the radicle and the chalaza, when 

 present, and the hilum, when the chalaza is not found, for the 

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



160. The embryo answers most important purposes in classi- 

 fication, since the whole vegetable kingdom has been divided 

 into three great classes founded on the varieties in structure of 

 the embryo. There have been found three varieties of embryo, 

 Monocotyledonous, Dicotyledonous, and Acotyledonous ; and it 

 has been observed that the vegetables arising from these differ- 

 ently constituted embryos, are distinct and peculiar in their con- 

 stitution and mode of growth. 



170. Fig. 161 exhibits the dicotyledonous embryo,, the dif- 

 ferent parts of which have been already pointed out. The 

 growth of this kind of embryo produces our forest trees, and all 



168. Is the apex of a seed opposite its base? Explain it. What often 

 determine the base and apex? How is the position of the radicle and 

 plumula determined? — 169. What three varieties of embryo ? 



