i66 
MORPHOLOGY OF MEMBERS. 
The typical form of roots is filiform and cylindrical ; their section is usually 
circular when not altered by external pressure. It is only when roots undergo 
a secondary increase in thickness, and serve 
as reservoirs for reserve-material, as in 
many Dicotyledons and some Monocoty- 
ledons, that the original filiform shape is 
changed into the fusiform or into tuberous 
swellings, as in the turnip, the tuberous 
roots of the dahlia, Bryojiia, Asphodelus, &c. 
Roots rarely form chlorophyll, and 
even then, as in Menyanthes, only in small 
quantities ; usually they are quite colour- 
less, not only when they grow in the 
ground, but also in water or air. 
A secondary basal growth appears never 
to occur in roots, as it does in many 
leaves and internodes when the regions near 
the apex have already been transformed 
Fig. 124.— Longitudinal section through a grain of maize 
(X about 6); c pericarp; n remains of the stigma; yj base of intO pCrmaUCnt tiSSUC. lutCrStitial grOWth 
the grain ; eg hard yellowish part of the endosperm ; eiu whiter _ 
less dense part of the endosperm; j-cscutellum of the embryo; bchiud thC apCX OftCU COUtlnUCS, hOWCVCr, 
ss its point ; e its skin ; k plumule ; lu (below) the primary 
root ; 7i/j its root-slieath ; 7f (above) secondary roots springing for a lOUg tlmC (lu LyCOpOdiaCCSS aCCOrd" 
fi om the first internode of the embryonal stem j-z". \ y r 
ing to Nägeli and Leitgeb) ; the extension 
of the tissue commences immediately behind the terminal part of the root formed 
of primary meristem, an arrangement by which the elongation of the roots in the 
ground is essentially assisted. 
(a) The primary root of the embryo of most Phanerogams gives the impression of 
being entirely exogenous, as if its apex were the actual posterior termination of the 
embryonal stem ; but its first origin is endogenous ; for the posterior end of the embryo 
is originally attached to the ' pro-embryo' or suspensor in Phanerogams, and the primary 
root is, at its first origin, covered by this^. There was formerly some doubt as to 
the endogenous origin of the primary root of Ferns and Rhizocarps ; but when it is 
observed that the root is not constituted as such until the apical cell has thrown off the 
first layer of the root-cap, it is evident that in this case also the apex of the new root 
lies from the first inside the tissue of the embryo^. 
(b) The origin of lateral roots in a mother-root is always on the outside of its 
axial fibro-vascular or plerome-cyhnder ; and the points where the new formation com- 
mences is — with a few exceptions among Phanerogams — on the outside of the vascular 
bundles, so that each bundle corresponds to a longitudinal row of secondary roots. 
There are however some diff"erences between the phenomena in Cryptogams (Ferns, 
Marsileaceae, and Equisetaceae ^) and Phanerogams, «uiz;., that in the former the roots 
originate from the innermost cortical layer or plerome-sheath which surrounds the 
^ A more exact account of this, according to Hanstein's researches on the formation of the 
embryo, will be given in Book II, on the Characteristics of Phanerogams. 
^ Compare the drawings of the embryos of Ferns and Rhizocarps in Book II, 
^ In Lycopodiacese (and according to Van Tieghem, Ophioglossacese) no lateral roots are 
formed in the mother-roots, the roots branching dichotomously, and the growing point which is 
enveloped by the root-cap splitting into two growing points, each of which forms its own root-cap 
•(see Fig. 138, p. 182). 
