ROOT. 



165 



m 



B 



hE 



carps and in Pleris aquilina. Roots are formed out of a secondary meristem much 

 further backwards- from the growing point, where 

 the tissue is already completely differentiated, in 

 older portions of stems, and especially when 

 mutilated, or when kept dark and damp. 



The order of development of the secondary 

 roots is, according to Nageli and Leitgeb, dis- 

 tinctly acropetal in the primary roots of Crypto- 

 gams, where they arise near the apex; new roots 

 are probably never formed in these plants be- 

 tween those already in existence. The same is 

 : probably always the case where roots are pro- 

 duced in the primary meristem or near the grow- 

 ing point of the stem, as in Pilularia, Marsilea, 

 ■Cereus, &c. But even where their origin is 

 further from the apex, as with the lateral roots 

 from the primary root of Phanerogams and 

 from many stems, such as the maize, they gener- 

 ally appear in acropetal order ; but owing to 

 subsequent disturbance roots may arise adven- 

 titiously, /. e. in abnormal positions, especially on 

 older primary roots of Dicotyledons. 



Secondary roots usually make their appear- 

 ance on the exterior of the fibro-vascular bun- 

 dles ; the fibro-vascular bundle of the secondary 

 root is then placed at right angles, or nearly so, 

 to those of the mother-root; the cortex is then 

 only incompletely 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 embryo that a complete con- 

 tinuity is possible in all the tissue-systems be- 

 tween stem and primary root ; but in Grasses 

 and some other Phanerogams the first root 

 arises so deep in the interior of the substance 

 of the embryo that it is covered, in the fully 

 developed embryo of the ripe seed, by a thick 

 layer of tissue (Fig. 124, ws), which is ruptured 

 on germination (Fig. 123, ivs), and is known by 

 the name of Root-sheath or Coleorhiza. Similar 

 formations occur also in the first lateral roots of 

 seedlings of Allium Cepa, and occasionally else- 

 where. But in other cases the secondary roots 

 which are formed deeper in the tissue simply split 

 the layers of tissue which cover them, and project from a two-lipped chink 



Fig. 123.— Germination of maize in the order /, //, 

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

 seen in front, in B from the side ; lu 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 ope:i 

 margins; * *' *'' the first leaves of the seedling 

 (natural size). 



