HAIRS {TRICHOMES). 



43 



B 



stems of Rhizocarps and of Pieris aquilina. Roots are formed much further back 



wards from the puftcfum vegetationis , where the 



tissue is ah-eady completely differentiated from 



a secondary meristem, in older portions of the 



stem, and especially when mutilated, or when the 



environment is dark and damp. 



The order of development of the secondary 

 roots is, according to Nageli and Leitgeb, dis- 

 tinctly acropetal in the mother-roots of Crypto- 

 gams, where they arise near the apex ; new 

 roots are probably never formed in these 

 plants between those already in existence in 

 the mother- root. The same is, probably, 

 always the case where roots are produced in 

 the primary meristem or near the punctum vege- 

 tatioiiis of the stem (as in Pilularia, Marsilea, 

 Cereus, &c.). But even where their origin is fur- 

 ther from the apex, as with the secondary roots in 

 the primary root of Phanerogams and in many 

 stems {Zea Mais, &c.), they generally appear in 

 acropetal order; but by subsequent disturbance 

 roots may arise adventitiously, /. e. in abnormal 

 positions, as especially on older primary roots 

 of Dicotyledons. 



Secondary roots usually make their appear- 

 ance on the exterior of the fibro- vascular bundles ; 

 the fibro-vascular bundle of the secondary root is 

 then placed at right angles, or nearly so, to those 

 of the mother-organ ; the cortex is then only in- 

 completely continuous with that of the latter, the 

 epidermis not at all so. The case is different in 

 the primary roots of embryos, which are formed 

 early and mostly so near the surface of the em- 

 bryo that a complete continuity is possible in 

 all the tissue systems between stem and primary 

 root ; but in Grasses and some other Phanero- 

 gams, the first root arises so deep in the interior 

 of the embryonal substance that it is enclosed in 

 the fully developed embryo of the ripe seed by a 

 thick sac-like layer of tissue (Fig. 114, ws), which 

 is ruptured on germination (Fig. 113, ws), and is 

 known by the name of Root-sheath (Coleorhiza). 

 Similar formations occur also in the first se- 

 condary roots of the germinadng plants of 

 Allium Cepa and occasionally elsewhere. But 

 the secondary roots which are formed deeper 



Fig. 113.— Germination of maize in the order /, //, 

 ///; A and B the embryo separated from /, in A seen 

 in front, in B from the side ; iv the primary root ; -ws its 

 root-sheath ; -w', w", -w'" secondary roots ; e the part 

 of the seed filled with endosperm ; k the plumule ; 

 sc scutellum of the embryo ; r r its open margins ; 

 b b' b" the first leaves of the embryo-plant (natural 

 size). 



in the tissue in other cases 



