204 HOFMEI6TER, OX 



found the small embryo-stem which Irmisch calls the 

 hypoeotyledonary axis, and which Clos calls the collet. The 

 place of origin of the root, i. c, the lower end of the embryo- 

 stem, is difficult to discover by direct observation, but may 

 safely be defined as the point at which in the lower end of 

 the very young embryo the cell-multiplication peculiar 

 to the root commences. Now, Avhether the young root of 

 the germinating plant has the appearance of an immediate 

 prolongation downwards of the embryo-stem (as is the case 

 with most dicotyledons, and with a few monocotyledons, 

 such as Jimcus, Allium, and Paris) — or "whether it (the 

 young root) breaks out from the interior of the lower end 

 of the embryo, as in the Palms and the Loranthacse — de- 

 pends simply upon the fact whether the place of origin, the 

 focus of cell-formation of the root, lies nearer to or further 

 from the lower end of the embryo. In both cases the 

 root is a main root. An adventitious root differs only in 

 the fact, that its longitudinal axis does not coincide with 

 the prolongation of that of the embryo, but forms with 

 the latter a considerable angle. For instance, the Orchi- 

 dese, the Pluviales, and especially (as Irmisch has well 

 observed), the Grasses, have no radicle, but only adventi- 

 tious roots. The distance from the surface of the place of 

 origin of adventitious roots is variable, being less in some 

 plants than in others. In the former case the surface of 

 the adventitious roots passes gradually into the cortical 

 layer of that portion of the plant from which they spring, 

 as may be observed in the pea when germinating. In the 

 latter the adventitious roots pierce through the outer cel- 

 lular cortical layers, throwing back those layers in the form 

 of a ring round the place of egress of the roots. The 

 absence of these characteristic collars (Coleorhizse), at the 

 base of the adventitious roots, is by no means unusual.* 

 Perns with creeping stems almost always have them, and 

 those with upright stems not unfrequently. It is well 

 known that all ramifications of roots, both those from main 

 roots and those from adventitious roots, are formed from the 

 outer surface of vascular bundles, and must therefore, with- 

 out exception, break through the bark. The reason why 



* See Irmisch's observations on ' Neottia nidus avis.' 



