144 



EXTERNAL CONFORMATION OF PLANTS. 



simply break through the layers of tissue which cover them, and then project from 



a two-lipped open chink. 



The typical form of roots is filiform and cylindrical ; their section is circular 



when they are not compressed from without. It is only when the roots undergo a 



subsequent increase in thickness, and serve 

 as store-houses, as in many Dicotyledons 

 and Monocotyledons, that the original fili- 

 form shape is changed into the fusiform 

 or into tuberous swellings (as in turnips, 

 tuberous roots of Dahlia, Bryonia, Aspho- 

 delus, &c.). 



Roots rarely form chlorophyll {e. g. 

 in Menyanthes), and even then only in 

 small quantities ; usually they are quite 

 colourless, not only when^they grow in the 

 ground, but also in water or air. 



A subsequent basal growth appears 

 never to occur in roots as it does in many 

 leaves and internodes when once the re- 

 gions near the apex have been transformed 

 into permanent tissue. Interstitial growth 

 behind the apex often continues, however, 

 for a long time (in Lycopodiacese accord- 

 ing to NageU and Leitgeb). The extension 



of the tissue commences immediately behind the terminal part of the root which 



has been formed of primary meristem — an arrangement by which the elongation 



of the roots in the ground is essentially assisted. 



Fig. 114. — I^ongitudinal section of the grain of Zea Mais 

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

 the grain ; <^ hard yellowish part of the endosperm ; e7Li whiter 

 less dense part of the endosperm ; jcscutelluin of the embrj'O ; 

 jj its point ; e its epidermis ; k plumule ; iv (below) the primary 

 root ; 7US its root-sheath ; lu (above) secondary roots springing 

 from the first internode of the embryo-stem it. 



(a) The primary root of the embryo of most Phanerogams gives the impression of 

 being entirely superficial, as if its apex were the actual posterior termination of the 

 embryonal stem ; but its first origin is endogenous ; for the posterior termination of the 

 embryo is connected with the ' pro-embryo ' in Phanerogams, and the primary root is, at 

 its first origin, covered by this. (A more exact account of this, according to the most 

 recent researches of Hanstein on the formation of the embryo will be given in Book II, 

 on the Characteristics of Phanerogams.) 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 embryonal tissue. (Compare the illustrations of the embryos of 

 Ferns and Rhizocarps in Book II.) 



(b) ^he Formation of Lateral Roots in a mother- root commences — as Niigeli and 

 Leitgeb have proved in the case of Cryptogams, and Reinke in the case of Phanerogams, 

 — in a layer of tissue which must be considered the outer layer of the plerome (or procam- 

 bium), and is called Pericambium. In Cryptogams the secondary roots originate in acro- 

 petal succession from definite single cells of the pericambium, which lie before the vascular 

 bundles, oblique divisions arising in them by which the three sides of the new apical cells 

 that lie behind are marked off; a transverse division follows immediately, by which the 

 first layer of the root-cap of the new lateral roots is separated. The apical cell of the 

 secondary root formed in this manner and already provided with a root-cap, forms new 



