112 COMPENDIUM OF GENERAL BOTANY. 



small or nearly, zero. The special anatomical structure of vascular 

 walls, namely their ability to withstand the high radial pressure 

 which proceeds from turgescent cells, can only strengthen our con- 

 clusions. FRANK ' calls attention to the fact that the large amount 

 of water required to supply the numerous leaves in the spring coin- 

 cides with an adaptive development of numerous large vessels and 

 tracheids. 



PEOTECTIVE SHEATH OE ENDODEEM. 



(CONCLUDING CHAPTER TO THE THREE FOREGOING ONES ON SPECIAL- 

 FUNCTIONS. ) 



C ASP ART'S term protective sheath as well as the term endo- 

 derm proposed by DE BARY are equally correct designations for the 

 tissue-system under consideration. Normally this structure occurs 

 in the roots ; it is also found frequently in stem-organs, even 

 in the leaves. Its functions are: 1. Mechanical; the delicate 

 elements of the leptome (albumen-conducting tissues) must be pro- 

 tected against injuries; the endoderm encloses these delicate 

 tissues in the form of a hollow or fluted cylinder. 2. In case of the 

 loss of the peripheral root-parenchyma and the root-epidermis it 

 functions in their stead by forming a protection against evapora- 

 tion as well as a protective tegumentary covering. 3. In case the 

 root remains intact it restricts the interchange of cell -sap in the 

 vascular bundle by its relative impermeability to water-solutions. 

 2 and 3 may be looked upon as those functions which would make 

 the term " endoderm " (inner tegument) especially applicable. 



The following statements will explain the nature of mechanical 

 injuries against which the endoderm forms a protection : Delicate 

 sieve-tube tissues are subject to torsions and stretchings due to 

 changes in turgor-pressure within the contiguous succulent paren- 

 chymatous tissue. If these lateral and longitudinal tensions pro- 

 ceeding from the parenchyma are to be harmless to the delicate 

 sieve-tube tissue, they must be enclosed by tissue-elements which 

 will neutralize or counteract these tensions. Thickening of the cell- 

 walls of the endoderm serves to supply the mechanical requisites ; 

 in extreme cases the ( c simple ' ' protective sheath receives a number 



Lehrbuch der Botanik, Leipzig, 1892-1893. 



