io6 



VASCULAR CRYPTOGAMS 



fore intermediate between the sheath-teeth ; these canals are of lysi- 

 genous origin — that is, they result from the disappearance of masses of 

 cells. Each bundle also contains a longitudinal air-cavity, or carinal 



cafial (sometimes called the ' essential air- 

 cavity '). The outline of the stem always 

 shows a number of alternate ridges and 

 furrows ; the ridges correspond to (or 

 are on the same radii as) the ' vascular ' 

 bundles, the furrows to the cortical lacunae. 

 This general description applies equally to 

 the primary vegetative stems, the fertile 

 unbranched stems, and the underground 

 rhizomes ; the branches of the barren 

 stems have no axial cavity or cortical 

 lacunae. In those species where the fer- 

 tile differ in structure from the barren 

 shoots, this appears to result from an 

 arrest of development of the latter. The 

 difference consists essentially in the 

 absence of chloroph3'll, the suppression 

 of branching, and the absence of sto- 

 mates, as well as in the greater develop- 

 ment of the leaf-sheaths. It is possible 

 to induce artificially the fertile shoots of 

 E. arvense (L.) to put out green branches 

 from the lower internodes, chlorophyll 

 being also formed in the main stem. 



The branches always spring from 

 within the leaf-sheath at its base, each 

 branch in the space between two teeth ; 

 they therefore correspond in number to 

 the ' vascular ' bundles of the stem, and 

 are always arranged in a whorl. The same 

 description applies to the roots. The 

 number of leaf-teeth and bundles is always 

 smaller on the secondary than on the pri- 

 mary axes, and these, as a rule, do not 

 again produce branches. In some species 

 even the primary stem never branches. 

 The whorl in which the branches stand is, however, not a true, but a 

 false whorl — that is, the phyllotaxis originally shows a regular spiral 

 one-third divergence ; but by subsequent unequal growth the insertions 



Fig. 8i. — E. viaxiinum. ^, portion of 

 stem (natural size). ?', i' , internodes ; 

 h, central cavity ; /, cortical lacunae ; 

 6", leaf-sheath ; a, a' . a", branches. 

 B, longitudinal section of rhizome 

 ( X 2). K, transverse diaphragm ; h, h, 

 cavities; g, 'vascular' bundle; /, 

 cortical lacunae ; S, leaf-sheath. C, 

 transverse section of rhizome. D, 

 union of ' vascular' bundles of two in- 

 ternodes, /, I ; K, node. {B — D, 

 diagrammatic.) 



