120 COMPENDIUM OF GENERAL BOTANY. 



fore they represent the weakest points m the mechanical structure 

 of the stem. At tliese points bending or breaking, due to lateral 

 forces, would most readily take place. The adaptations which 

 occur in the plant creation to protect this process of growth by 

 intercalation were also revealed by Schwendener's investigations, 

 concluded in 1874. 



There are two essential means for securing this protection. In 

 the one case it is the employment of a special tissue-system with 

 specific physiological properties. As is known, tjq^ical mechanical 

 cells are eitlier lifeless or at least incapable of elongating after they 

 have once acquired their extensive wall-thickenings; they can 

 therefore not exist in growing organs. This special mechanical 

 tissue is the collenchyma, which we learned to know in the chapter 

 on the cell. It is very readily recognized by the characteristic 

 thickenings of the angles of the cell-wall. In spite of these thick- 

 enings it is capable of growth. This tissue has approximately the 



Fig. 67. — Cross-section of the leaf-sheath of Brachypodium silvaticum. 



(After Haberlandt.) 



same firmness (extensibility) as the typical mechanical tissue, but is 

 peculiar in that it becomes permanently elongated (without tearing) 

 when subjected to a slight pulling force (Ambronn). Collenchyma 

 frequently serves to strengthen the growing internodes of dicoty- 

 ledonous stems {Compositce^ Umbelliferce., Lahiatece., etc.). This 

 arrangement may in many cases be combined Avith the second form 

 of protective adaptation, which we shall now consider. 



The envelopment of the growing stem by supporting tubes is 

 of very frequent occurrence among monocotyledons, less frequent 



