STEREOM AND MESTOM. 191 



528. The strength of other tissues besides bast has been meas- 

 ured ; thus Ambronn assigns to collenchyma a breaking-weight of 

 12 kilograms per square millimeter, and these cells become per- 

 manently elongated under a weight of from 1.5 to 2 kilograms. 



Haberlandt found that the breaking-weight of the internal 

 " thread " of the common graybeard lichen, Usnea barbata, is 

 1.7 kilograms per square millimeter, but that this thread could be 

 stretched to double its length before breaking. The breaking- 

 weight of cotton fibre is calculated to be between 18 and 20 

 kilograms per square millimeter, and that of the seed-hair of 

 Asclepias Syriaca not far from 40 kilograms. 



529. Examination of any of the figures of fibro-vascular 

 bundles given in Part I. shows how well their elements are dis- 

 tributed in order to secure the greatest strength with economy of 

 material. To the elements which impart strength to a bundle 

 Schwendener has given the name stereom ; to the other parts of 

 the bundle, mestom ; thus the fibres are stereom elements, the 

 ducts are mestom elements. 



530. The striking adaptations 1 of the fibro-vascular bundles 

 to serve as light and very strong building materials in the plant 



1 The following table from Schwendener, with a few illustrative examples, 

 is given to serve as a guide to the student in tracing out a few of these adapta- 

 tions : 



DISTRIBUTION OF MECHANICAL ELEMENTS IN MONOCOTYLEDONS. 



I. In cylindrical organs. 



1. System of subepidermal nerves of bast. Simple fascicles of bast lie 



under the epidermis. 

 First type. Arum, Arisaema. 

 Second type. Petioles of Colocasia and Alocasia. 



2. System of compound peripheral girders. Subepidermal fascicles of bast 



unite with those which lie more deeply to form girders in which 

 the "web" or binding-tissue is partly mestom, partly parenchyma. 



Third type. Stems of Scirpus csespitosus and Eriophorum alpinum. 



Fourth type. Stems (above ground) of Cyperus alternifolius. 



Fifth type. Stems of Schcenus nigricans. 



Sixth type. Stems of Juncus effusus. 



Seventh type. Carex lupulina. 



Eighth type. Scirpus lacustris. 



Ninth type. Isolepis pauciflora. 



Tenth type. Cladium Mariscus. 



8. System characterized by a nerved hollow cylinder, the nerves of 

 which are united with those at the epidermis. 



Eleventh type. Many grasses ; e. g., Alopecurus pratensis. 



Twelfth type. Panicum Crus-galli. 

 4. System of peripheral bast-fascicles strengthened by mestom. 



Thirteenth type. Zea Mais. 



