192 PHYSIOLOGICAL CLASS IF ICATtOISt OP TlSSUfe^. 



are seen plainly when the distribution ol' the bundles in the stems 

 of monoeot} ledons is examined in eross-section. In manj- eases 

 the shape of the section of the bundle is nearly that of the 

 well-known " 1 " or " H " beam or girder. In the most clearly 

 marked instances the stereom portion is well developed on 

 both sides of the mestom, and thus forms the "flanges" or 

 "plates," while the mestom is the "web;" the stereom has 

 therefore to bear either compression or tension, according to 

 the bending of the part. It will further be observed that in all 

 cases the beam is placed with respect to the rest of the stem, so 

 as to insui-e the greatest efHeiency of the stereom portion. 



But it is only upon a careful exavniuatiou of the many methods 

 of arrangement of the stereom and mestom in the bundles 

 of diverse forms of dicotjdedonous stems, together ^ith an ex- 

 amination of the arrangement of the bundles themselves with 

 respect to the surrounding tissues, that the adaptations of the 

 various elements to strength can be full}' appreciated. 



The modes of distribution of the stereom and mestom met 

 with in monocotyledons are so numerous that thej- cannot be 

 reduced to a few types ; their diversitj' is so great that they can 

 only with difficulty be brought into any system of classification. 



5. System of subcortical fibro-vascular bundles with strongly marked 



bast development. 

 Fourteenth type. Bambusa species. 

 Fifteenth type. Palms. 

 Sixteenth type. Yucca. 

 Seventeenth type. Musa. 

 Eighteenth type. Maranta. 



6. System of subcortical fibro-vascular bundles united tangentially. 

 Nineteenth type. Juncus Gerardi. 



7. System characterized by a simple hollow cylinder with imbedded or 



attached fascicles of Mestom. 

 Twentieth type. Commelynacese. 

 II. In bilateral organs. 



1. System of subepidermal girders. 

 First type. Leaves of Cyperus. 



Second type. Middle part of leaves of Zea. 

 Third type. Leaves of Musa. 

 Fourth type. Leaves of Tradescantia. 

 Fifth type. Leaves of Pardanthus. 



2. System of internal girders. 



Sixth type. Leaves of Cypripedium. 

 Seventh type. Petiole of Aspidistra. 



3. System of complex girders ; subepidermal nerves of bast combined 



with interior girders. 

 Eighth type. Petioles of many palms. 



