OUTIiR CELLS AND TISSUES. 41 



elongated cells (prosenchyma), the walls of which become greatly 

 thickened (sclerised), and at times deeply pigmented, usually 

 a brown colour; as soon as the thickening is completed the 

 protoplasm leaves the cell-cavity and passes through " pits " in 

 the walls into other cells. A transverse section across a patch 

 of sclerenchyma shows rather irregular rounded or polyhedral 

 elements with very thick walls, the latter being perforated here 

 and there by narrow "pits" joining the cavities of adjacent 

 fibres. The walls also show concentric striations, pointing to 

 the fact that the various thickening layers have been laid down 

 at different times (c/. growth of the cell-wall by accretion. 

 Chap. iv.). On treatment with iodine solution the fibres stain 

 a yellowish-brown. In longitudinal sections of stems or roots the 

 fibres are seen to be elongated fusiform elements, not composed, 

 as would appear, of single cells, Init of several which have united 

 end to end, the intermediate end-walls becoming absorbed. 

 The fibres join one another obliquely, and have tapering ends. 



Note. — Sclerenchyma may be studied in transverse and longitudinal 

 sections of the stems of Zea mais, Piniis, and the rhizome or petiole of 

 Ptei-is aquilina. Tilia and Euphorbia stems are also good. In Zea mais 

 the fibres are arranged round the fibro-vasoular bundles; the "pits" in 

 the walls are well seen in transverse sections of the leaf of Sanfevieria and 

 of the stem of Euphorbia. In the stem of Finns the sclerised elements are 

 the bast-fibres ; they are of an oval, flattened shape in transverse section. 



Fig. 27. — A Small Patch of Sclerenchyma from the Leaf of Sa7ise- 

 vieria, seen in Transverse Section. — Note the laminated structure 

 of the walls of the fibres and the "pits" connecting their cavities. 

 The large cells surrounding the patch are cells of the mesophyll. 



