114 COMPENDIUM OF GENERAL BOTANY, 



sheath, that is, the eliaracteristic thickening of the cell-walls next 

 to the leptome, and the thin-walled cells (passage-cells or transit- 

 cells) which form the ' ' channels ' ' of communication that is, points 

 of interchange of liquids (cell-sap, etc.), between the parenchj-ma 

 and vessels, Vicia root (Fig. 58) shows the frequent, but not 

 readily understood, formation of the protective sheath as it usually 

 occurs in dicotyledonous roots and in the stems of water-plants. 

 The thin-walled cells of the protective sheath are here either j9c<r- 

 tiaUy or totally suberized ; in the former case onl}^ the radial walls 

 or sometimes only tlie middle portions or l)ands of the radial walls ; 

 even with such partial suberization the bands form a continuous 

 hollow cylindrical network. In its mechanical function this cylin' 

 drical network may be compared to the protective net-work of 

 rope enclosing a balloon (Schwendener). In the case of the vas- 

 cular bundle of the root the pulling tensions are of course on 

 the outside, and not on the inside, as in the balloon. The follow- 

 ing will serve to explain the dark spot (" Caspary's dot ") seen on 

 the radial walls in cross-section (Fig. 58). They are local thicken- 

 ings of the cell-walls (Caspary). The suberized bands of the cell- 

 wall are only slightly extensible, and hence only slightly contractile. 

 The unsuberized walls of the neighboring cells and the unsuberized 

 portions of the sheath-cells are highly elastic and become expanded 

 by the turgor-force, but contract again as soon as turgor is sus- 

 pended ; the less elastic portions can only adapt themselves to this 

 contraction by forming wavy foldings (Schwendener). These wavy 

 foldings can be seen in tangential sections. The dark spot seen in the 

 cross-section is only the oiJtical effect due to the membranous folding. 

 When the tangential walls are unsuberized, the above-mentioned 

 "limiting " or " bounding " function is excluded (see 3, page 118). 



Roots of ferns gromng upon walls and rocks, and hence exposed 

 to great variations in the supply of water, have enormously devel- 

 oped protective sheaths. (For particulars see Schwendener' s com- 

 munication on Protective Sheaths and their Strengthenings.)' 



Without in any way interfering with subsequent statements, we 

 shall here briefly consider Schwendener' s'' more recent investiga- 

 tions on the mestome-sheaths of gramineous leaves. The phytoto- 



'Die Schutzscheiden und ibre Yerstarkungen : Abbaudlungen der Berliner 

 Akademie, 1883. 



- Sitzungsber. der Berl. Akad. 1890. 



