158 MECHANICAL SYSTEM 



Tlie thickened walls of collenchymatous cells have a high refractive 

 index, and are hence particularly conspicuous in a transverse section 

 on account of their brilliant appearance. When treated with chlor- 

 zinc-iodine, or with iodine and sulphuric acid, they assume a bright 

 blue colour, and are thereby shown to be more closely related even 

 than the walls of bast-fibres to so-called pure cellulose, in respect of 

 chemical composition. According to J. Conn, collenchymatous walls 

 contain the remarkably high proportion of 60-70 per cent, of water, 

 whereas lignified bast- and wood-elements contain only 20-40 per 

 cent. The water-content rises in the case of the individual cell-wall, 

 as one passes from its inner surface to the middle of each thickened 

 edge. Judging from the mode of contraction under the influence of 

 absolute alcohol and other dehydrating agents, this water of imbibition 

 is chiefly interpolated in the radial direction, less in the tangential 

 and least of all longitudinally. Thus J. Colin observed, in the case of 

 Enpatorium cannabinirm, contractions in the radial direction of 22-33 

 per cent., in the tangential of 7-12 per cent., in the longitudinal of 

 not more than '5-75 per cent. The smallness of the longitudinal 

 contraction had previously been noted by Ambronn ; it proves that the 

 ultimate particles of the membrane are most firmly knit together in 

 the longitudinal direction, that is to say in the direction in which the 

 mechanical strength of the membrane is most severely tested. 



In contrast to bast-fibres, collenchymatous cells always retain 

 their living contents, even where they serve as the permanent 

 mechanical elements of fully developed organs. They also frequently 

 contain chlorophyll corpuscles, though these are generally but few in 

 number. 



4. Schrenchijmatons cells [or Sclereides]. 92 



All stereides which are not prosenchymatous will in the present 

 work be termed sclerenchymatous cells or sclereides (Tschirch). Such 

 cells are largely employed by plants for a number of local mechanical 

 purposes, and accordingly exhibit considerable diversity as regards 

 their morphological characters. Mote ' or less isodiametric scleren- 

 chymatous cells (stone-cells or brachvsclereides) occur particularly in 

 the cortex of Dicotyledonous woody plants, where they usually owe 

 their origin to the secondary sclerosis of thin-walled parenchyma. 

 They generally serve to increase the incompressibility of the bark ; 

 their action may be compared to that of the sand which a mason uses 

 to increase the tenacity of his mortar, or to that of the powdered 

 glass which is added to gutta-percha in order to render it less com- 

 pressible. The cortex of the young twigs of many deciduous trees 

 {Querms, Juglans, Garpinus, Bclula, Fraxinvs, etc., etc.) contains 



