66 



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, Bryonia, 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 

 into permanent tissue. Interstitial growth 

 behind the apex often continues, however, 

 for a long time (in Lycopodiaceae accord- 

 ing to Nageli 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-cylinder ; 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 differences between the phenomena in Cryptogams (Ferns, 

 Marsileaceae, and Equisetaceae '^j and Phanerogams, 'z^/z., that in the former the roots 

 originate from the innermost cortical layer or plerome-sheath which surrounds the 



Fig. 124. — Longitudinal section through a grain of Inaiae 

 (X about 6); c pericarp; n remains of the stigma; fs base of 

 the grain ; eg hard yellowish part of the endosperm ; ew whiter 

 less dense part of the endosperm ; sc scutellum of the embryo ; 

 ss its point ; e its skin ; k plumule ; w (below) the primary 

 root ; ws its root-sheath; -w (above) secondary roots springing 

 from the first internode of the embryonal stem st. 



^ 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. 



2 Compare the drawings of the embryos of Ferns and Rhizocarps in Book II, 



3 In Lycopodiacece (and according to Van Tieghem, Ophioglossaceae) 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), 



