284 THE BIOLOGY OF FLOWERING PLANTS 



collenchyma ; (c) to the presence of woody or lignified tissue, 

 as in the various elements of the wood, but more especially 

 in the wood fibres, and in the sclerenchyma cells of the 

 ground tissue, cortex, and bast. 



Turgor. — The action of turgor and the presence of 

 collenchyma is of special importance in growing regions. 

 The effect of turgor is demonstrated by the behaviour of 

 any young stem or leaf on wilting — the flaccidity is due to 

 collapse of the cells, following on loss of water ; the rigidity 

 in the fresh state is consequently the effect of turgor pressure, 

 the inflation of the cells by water, and depends on the 

 osmotic pressure of the solutes of the cell sap. Another 

 demonstration may be given by soaking a young stem, the 

 scape of the dandelion is excellent material, in a strong salt 

 solution ; plasmolysis occurs, and the rigid, brittle stem 

 becomes quite flaccid. At the same time its length 

 diminishes by 2 to 5 per cent., showing that the young cell 

 membranes are elastically expanded by turgor pressure in 

 the fresh condition. 



Turgor pressure produces rigidity in exactly the same 

 way in which aerostatic pressure produces rigidity in a 

 sausage balloon. Its effect is heightened by the fact 

 that the stem or leaf is built up of millions of small cells. 

 It is also frequently increased by the occurrence of tissue 

 tensions. Again we may take the example of the dandelion 

 scape. If a strip is cut it rolls up into a coil ; if this is 

 plasmolysed it straightens out and becomes limp. The 

 inner tissues are more extensible than the outer, wliich act 

 as a resistance, against which the inner tissues try to expand, 

 so that in the intact stem the rigidity is thus increased. 

 The same feature, in less marked degree, may be seen in 

 most young stems which, if split longitudinally, bend away 

 from the centre. 



Collenchyma. — The turgor effect is produced in any 

 living cell plentifully supplied with water, and exists also 

 in the cells of the collenchyma. This tissue consists of 

 parenchymatous or prosenchymatous cells, the walls of 

 which^are usually much thickened by the deposition of 



