MOVEMENTS OF IRRITATION. 523 



afforded, e.g., by strips from the stems of Levisticum officinale and 

 Foeniculum officinale. 



1 See Schwendener, Das mechanische Princip im anatomivchen Bau der 

 Honocotylen, Leipzig, 1874. 



2 Further literature : Ambronn in Pringsheim's Jahrbticher, Bd. 12 ; Haber- 

 landt, Physiol. Pflanzenanatomie, Leipzig, 1884, p. 96 ; Tschirch in Prings- 

 heim's Jahrbiicher, Bd. 16 ; Lukas, Sitzungsber. d. Akad. d. Wiss. zu Wien, Bd. 

 85 ; fconntag, Landwirthscld. Jahrb., Bd. 21. 



196. The Arrangement of the Mechanical Tissues in Structures 

 Resistant to Flexion Tension and Pressure. 



We shall here consider a series of objects, the majority of 

 which I have myself investigated, and which serve well for study- 

 ing the arrangement of the stereome. We only need to submit 

 transverse sections of the structures to microscopic observation, 

 and we begin with organs constructed to resist flexion. 



In the leaf stalk of Begonia the mechanical tissue consists of a 

 well-developed ring of collenchyma lying immediately below the 

 epidermis. This encloses parenchyma, in which are distributed 

 the vascular bundles. In the stem of Lamium album, bands of 

 collenchyma run in the four angles of the stem, forming two 

 pillars combined crosswise. The transverse section of the stem of 

 Falcaria rivini shows a large pith, the circle of vascular bundles, 

 and, below the epidermis, strands of collenchyma, alternating with 

 assimilatory parenchyma and acting as mechanical tissue. 



In the flower scapes of Papaver, Armeria maritima, Lychnis 

 viscosa, and Anthericum ramosum the mechanical tissue is in the 

 form of a closed ring of sclerenchyma. Between this and the 

 epidermis occurs green tissue, while the vascular bundles are 

 disposed within. On microscopic examination of a transverse 

 section of the haulm of Juncus glaucus, we perceive under the 

 epidermis green tissue and sclerenchyma bundles, which alternate 

 with one another. We further see distributed in the ground 

 tissue wide air-canals, and numerous vascular bundles containing 

 some fairly wide vessels, and provided both on the inside and 

 outside with a layer of bast fibres. The structure which we 

 observe on examination of a cross section of the haulm of Sesleria 

 coerulea is easy to understand, and we are here specially in- 

 terested in the layers of bast in the vascular bundles, since 



