600, Section of an Acrogenous stem of Tree- 

 Fern (Cyathea), showing the vascular bundles 

 imbedded near the circumference of the cel- 

 lular mass. 



722. THALLOGENS are the lowest 

 in the scale of rank, having no true axis 

 and no other tissue than parenchyma, 

 which grows in threads or in mass in 

 all directions. The apparent stems 



599, Various kinds of vessels in a wood- 

 fiber of Bamboo or Kattan. tf, Cells of paren- 

 chyma ; 6, annular cells ; c, spiral vessels ; 

 <?, porous duct ; , wood-cells. 



(stipes), if any, support the fructification only (sea- weeds, lichens, mushrooms, puff- 

 balls, frog-spittle, mildew). 



723. THE STRUCTURE OP ROOTS presents few deviations from that of the stems 

 to which they severally belong, being exogenous in Exogens, endogenous in En- 

 dogens, etc. In the former class the central pith disappears, its place being occu- 

 pied mainly by vascular ducts, and the liber, if any, has no bast -cells. 



601 602 724. THE PIBRILL^ and pileor- 



hiza should, however, be mentioned 

 as peculiar in the structure of the 

 root The former are produced by 

 millions, clothing the delicate epi- 

 dermis of the young rootlets as with 

 cottony down, especially in light 

 soils. They usually consist of a 

 single cell of the epidermis extended 

 as seen in figure 601. They are the 

 true absorbents, the mouths of the 

 growing plant. 



725. THE PILEORHIZA. The mi- 

 croscope shows that the extreme, 

 advancing point of the delicate, -grow- 

 ing fibers is not thrust naked against 

 the opposing soil, but is covered 

 601, Extremity of the rootlet of Maple, with with a cap called pileorhiza (pileus, 

 fibrillae and (s) pileorhiza. 602, Two plants of a cap, rMza, root), which consists of 

 Lemna minor (Duckmeat). a, Their pileorhiza. older, hardened cells, behind which 

 are formed the new cells. In the Duck-meat the pileorhiza is lengthened into a 

 sheath. 



726. THE MANNER OP GROWTH IN THE ROOT is not like that of stems, by the 

 extension of parts already formed, but simply by the addition of new matter at the 



