ROOT. 
lös 
J 
carps and in F/en's aquilina. Roots are formed out of a secondary meristem much 
further backwards from the growing point, where 
the tissue is ah-eady 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 Nägeli 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 Pihdaria, Marsi/ea, 
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, i. 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, ws), 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 spUt 
the layers of tissue which cover them, and project from a two-lipped chink. 
Fig. 
Ill; 
23.— 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 ; 
■ZDS its root-sheath ; -w', tu", ju"' 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 seedling 
(natural size). 
