152 ACADEMIC BOTANr. 



LESSON XXX. 



ROOT— STEM. 



371. Hoot. 372. The two cones. 373-375. Koot-growth. 376. 

 The Stem : Acrogenous ; 377. Endogenous ; . 378. Exogenous. 379. 

 Pith. 380. "W ood- wedges ; 381. Section. 382. Sap-wood, Heart- 

 wood. 



371. The Root has the same structure as the stem to 

 which it belongs, and usually imitates it. In Endogens 

 the radicle dies early ; its place is supplied by adventitious 

 roots -springing 'around the collum or often 

 above it. In the Pines the radicle persists, 

 forming a long tap-root. In the Oaks and 

 other Exogens it persists also, but becomes 

 solvent. The root has no tracheae nor medul- 

 F.G. 2 2 2.- laryrays. 

 Toung rm.t of 372. The higher plant consists of two op- 



Seedhng M«ple - , ^- ^ ^ i_ j.1 



{Acer ccmpes- posed vegctativc cones, one subterranean, the 

 rhiza; /! fibrils ; Other supcrterrancan ; their point of union and 

 ^' departure is the collum, which is usually a 



mere mathematical point, having position without dimen- 

 sions : the root, therefore, is a subterranean leafless tree ; 

 the stem a superterranean leafy one. It has been said that 

 an exogenous tree, with all the needful conditions of cK- 

 mate and soil, would exhibit the perfect model ; its root, 

 rootlets, and fibrils exactly corresponding to the trunk, 

 branches, and leaves ; with this marked difference, how- 

 ever, that the root elongates only at the extremity of its 

 branches, whereas the stem and its branches elongate 

 throughout. 



373. The young root, in all plants, is at first purely cel- 

 lular. It is furnished with fibrils (Fig. 222,/), which are 

 prolongations of the outermost cells, and whose office is to 

 increase the absorbing surface. They are fine, soft, thin- 

 walled, without openings' or pores. As the rotit grows, 

 wood-cells and vessels appear, and in the higher plants 



