TISSUES AND SIMPLE ORGANS. 115 



mist in the study of the leaf anatomy frequently observes that the 

 vascular bundles, especially of the monocotyledons, are supj)lied 

 with a thickened endoderm {mestome- sheath)^ and in addition, im- 

 mediately outside of the same, a " parenchymatous sheath," which 

 is usually chlorophyll-bearing and which we have already referred 

 to in the discussion of the conducting system. As examples in 

 which this occurs we may mention Poa lyratensis and JBambusa 

 vuJgaris. In the absence of the mestonie- sheath it frequently 

 happens that the parenchyma-sheath, although belonging to the 

 conducting system, shows modifications in its structui-e (thickening 

 of walls, suberization, etc.) which enable it to perform the function 

 of the endoderm. Vice versa it may happen that in the absence of 

 the endoderm a part of the mestome-cells, especially a crescent- 

 shaped group of the leptsome -parenchyma, may become thickened 

 and so perform a mechanical function. 



V. PEOTECTION OF THE MERISTEMATIC AREAS OF 

 THE PLANT-BODY. 



Young, undeveloped leaves perform a service for the apical 

 area of the stem similar to that which the root-cap performs for 

 the root-apex. The rolling-in of the young leaves of ferns serves 

 a purpose similar to that which sinking-in of the apical area does 

 in the algae {Fucus^ LawenckC). The leaf-sheatlis of grasses and 

 Equisetd'^ and the sheath enclosing the growing jieduncle of Arme- 

 ria, serve a similar function for the internodes which they enclose 

 as does the collenchyma in firm growing tissues. With these 

 statements we have hastily sketched the teleological significance of 

 the subject under discussion. In all cases we are concerned with 

 the protection of delicate meristematic tissues against definite 

 mechanical injuries as well as against injuries in general. 



The special consideration of individual cases will give us an 

 opportunity to mention important facts pertaining to the develoD- 

 ment of the leaf. 



{a) The Protection for Terminal Meristernatic Areas of the 

 Plant-hody. 



a. Protection of the Root-tip. 



The growing root-tip forcing its way between particles of soil is 

 supplied with a specific protective organ^ namely, a bell- shaped 



