EMBRYO. 57 



different purposes, although Bernhardi has stated that 

 in many cases the limits between the two can be pretty 

 well made out, and to which the term collum has been 

 applied. 



308. Such as we have described are the general rules 

 respecting the embryo and its relations, &c. in a Di- 

 cotyledonous plant, but in Monocotyledons they are 

 somewhat different. 



309. When we examine the embryo of the latter 

 class, in the greater number of cases we find it a solid, 

 uniform, somewhat cylindrical body, in which, when 

 divided, no distinct radicle or plumula can be seen, 

 they being incorporated with the one cotyledonary sub- 

 stance; in some, however, like those of Aroidese and 

 Graminese, they are more distinct, but the whole embryo 

 in these two cases is somewhat modified. 



310. When such an embryo is placed under the con- 

 ditions of germination, the lower end of it elongates 

 and opens, emitting the radicles, 



which have their origin from the Fig. 51. 



internal substance of the minute 

 radicular extension of the em- 

 bryo, and not, as in the other 

 case, by an immediate extension 

 or lengthening of this axis, so 

 that here they are enclosed at 

 their origin by a sheath formed 

 out of the structure through 

 which they have to pass; whilst in a Dicotyledonous 

 embryo, the radicle is naked. In consequence of these 

 facts Monocotyledonous plants have been termed endor- 

 rhizal, having the radicle in a sheath, and Dicotyledons, 

 exorrhizal, the radicle being destitute of one. 



311. There are other modifications, however, of the 

 mode of protrusion of the radicle ; and even in Dicotyle- 

 donous plants the radicle, as remarked by Decandolle, 

 may be made endorrhizal by cutting off the extremity of 

 the original radicular axis, so that the internal substance 

 of it must give out new ones : but notwithstanding what 



