COLLENCHYMA 157 



undergoing elongation must therefore make use of a mechanical tissue 

 which is itself capable of active extension by means of growth. The 

 tissue which satisfies these requirements is collenchyma. The fact 

 that collenchyma regularly forms the skeletal system of growing 

 organs does not preclude it from serving as the permanent mechanical 

 tissue in many fully grown herbaceous structures (petioles, etc.). On 

 account of its flexibility and power of growth collenchyma is further 

 utilised in motor organs, such as the foliar pulvini of Legumixosae, 

 the nodes of Grass-haulms, etc. 



With reference to the shape of the individual elements, one may 

 distinguish between prosenchymatous and parenchymatous collenchyma 

 cells. The former sometimes attain to a considerable length (up to 

 2 mm.) ; like bast-fibres, they are often chambered and provided with 

 slit-shaped, vertically elongated pits. 



The walls of typical collenchymatous cells are thickened in a highly 

 characteristic manner. The deposition of thickening layers, namely, 

 instead of being uniformly distributed, is restricted to the edges of 

 the cells, or is, at any rate, much more pronounced along the edges 

 than in any other part of the wall (Fig. 49 b). This peculiarity 

 is closely connected with the fact that collenchyma serves as the 

 mechanical tissue of growing organs. The presence of unthickened or 

 at most slightly thickened strips between the thick portions of the 

 walls entails a twofold advantage. In the first place this mode of 

 construction endows the whole tissue with greater plasticity, since the 

 individual cells are not rigidly joined together, a circumstance which is 

 particularly advantageous in connection with the growth in thickness 

 of the organ concerned. The absence of thickening layers from 

 certain regions of the wall also facilitates diffusion between neighbour- 

 ing cells, and hence greatly accelerates interchange of nutritive 

 material. 



The typical form of collenchyma which has just been described is 

 termed " angular " collenchyma (Eckencollcnchym ) by Carl Muller. Where 

 the thickened edges of the cells contain intercellular passages (e.g. 

 spp. of Pdasites and other Compositae, Fig. 49 i>), the same author 

 speaks of "tubular" collenchyma (Liichncollcnchym). If the thickening 

 is confined to the tangential walls, and is uniformly distributed over the 

 latter, a "lamellar" collenchyma (PlattcncoUcncliyrii) results (Astrantia, 

 Sanguisorba, etc., Fig 49 c). If finally the walls are thickened on all 

 sides, and if the innermost pellicle is sharply differentiated, while the 

 middle lamella, on the contrary, is indistinguishable, one may use the 

 term "cartilaginous" collenchyma (KnorpclcoUenchym). Collenchyma 

 in general may indeed not inaptly be compared to the cartilage of the 

 animal body. 



