COMPOSITION OF FI BRO-VASCULAR BUNDLES OF PLANTS. 389 



cells. This layer is the true sheath of the bundle, and has 

 been particularised under various names (Schutzseheide/ 



G 



Pig. ]. — Transverse section of bundle of Pteris aquilina (after Sachs). X, 

 xylera ; P, phloem, a, bundle-sheath ; b, phloem-sheath. G, ground tissue. 



Vaginalschicht,^ &e.); but we shall always refer to it as the 

 "bundle sheath." The inner layer {h) consists of parenchy- 

 matous cells like those of the outer layer, but differing from 

 them in that they contain starch, and to this layer Riissow^ 

 has given the name of Phloem sheath. These two layers, 

 then, together make up the true sheath tissue or koleochyma 

 of the bundles of this fern. When the bundle sheath is in- 

 vested by sclerenchymatous cells we may regard these cells 

 as forming yet another covering for the bundle, and to this 

 the name of " external sheath" may be applied (Aussen- 

 scheide, Russow). 



In Lycopodium (chamsecyparissus) we meet with these 

 same layers, but their arrangement has been modified as a 

 consequence of the fusion of the fibro-vascular bundles to 

 form an axial cylinder. Here the bundle sheath, which is 

 very well marked, surrounds the whole of the vascular tissue, 

 from which it is separated by a phloem sheath (Hegelmaier), 



^ Caspary, ' Jahrb. f. Wiss. Bot.,' i and iv. 



2 Sachs, ' Lehrbuch der Botanik,' 4te Aufl., p. 126. 



3 'Vergl. Uuters.' 



