StiSKEUM AJtD MESTOM. 19l 



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

 ured ; thus Ambronu assigns to eollenchyma a breaking- weight ol' 

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

 manentlj' elongated under a weight of from 1.5 to 2 kilograms. 



Ilaberlandt found that the breaking-weight of the internal 

 " thread" of the common graj'beard 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 Sjriaca not far from 40 kilograms. 



529. Examination of anj' of the figures of flbro-vascular 

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

 tributed in order to secure the greatest strength with econom}- of 

 material. To the elements which impart strength to a bundle 

 Schwendener has gi\'en 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-' of the fibro-vascular bundles 

 to serve as light and ver3' strong building materials in the plant 



^ 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 : — 



DiSTRIDUTION 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, Arisaima. 

 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 cfespitosus and Eriophomm alpinum. 



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



Fifth type. Stems of Schoenus nigricans. 



Sixth type. Stems of Juncus effusus. 



Seventh type. Carex lupulina. 



Eighth type. Scirpus lacustris. 



Ninth type. Isolepis paucifiora. 



Tenth type. Cladium Mariscus. 



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



which are united with those at the epidermis. 

 Eleventh type. Many grasses ; c. </., Alopecurus pratensis. 

 Twelfth type. Panicum Crus-galli. 



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

 Thirteenth type. Zea Mais. 



