726 RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 



plasm disappears from its interior, and the narrow space of the cell-cavity becomes 

 filled with air, or less often with a watery fluid. The cell can then no longer 

 continue its growth, neither can it serve to take up and conduct food nor to 

 manufacture organic compounds; it cannot be employed in transformations and 

 transmission of materials, and has exclusively an architectural significance. It is 

 excellently adapted, however, to the task thus assigned to it. Its strength and 

 elasticity are indeed extraordinary. It has been estimated that the bearing capacity 

 of hard bast amounts to between 15 and 20 kg. to the sq. mm. in cross section, and 

 is therefore equal to that of wrought iron; indeed the bearing capacity of many 

 species of plants is even equal to that of steel. Hard bast has this advantage over 

 iron, that it is far more extensible and consequently less subject to breaking. From 

 the consideration of all these properties it becomes evident why the hard bast of 

 many plants has been used by man to such advantage in the manufacture of fabrics, 

 string, ropes, and the like, since very remote times. 



Woody fibres, also known as libriform cells, differ very little from hard bast 

 cells. Whilst hard bast forms one of the most important constituents of the cortex, 

 the woody fibres form an essential element in the wood of those stems which 

 annually add a new layer to the already existing wood, thus increasing in 

 circumference and exhibiting annual rings in cross section. Their length varies 

 between 0'3 and 1'3 mm., so that they are somewhat shorter than the fibres of the 

 bast. Their walls are as a rule strongly lignified, but in other respects it is 

 impossible to draw a sharp line between the two forms of cells. When a woody 

 stem has grown in thickness and has developed bark on its periphery, the role 

 played by the hard bast in the cortex is evidently at an end; the woody fibres 

 then assume the tasks which in the young shoots are allotted to the hard bast, 

 and they might therefore be called the hard bast cells of the wood. 



In many plants a special form of mechanical cell-tissue is developed, known 

 as collenchynia. The cells which compose it are elongated and connected with one 

 another just like hard bast cells, but they differ from these and from the woody 

 fibres in the fact that their walls are unequally thickened. Where three or four 

 of these cells adjoin one another by their long sides the walls are very thick, but 

 in places the wall common to two neighbouring cells remains thin; the whole of 

 the tissue may be compared to a building in which thick main walls alternate 

 with thin partitions which are strengthened here and there with quartering, and 

 attain a great supporting capacity. A further distinction from hard bast cells and 

 woody fibres consists in the fact that living protoplasm remains in the interior of 

 coUenchymatous cells in which chlorophyll-corpuscles are often embedded; more- 

 over, this protoplasm can draw some of the materials necessary for growth through 

 the thin places in the walls from the surronding tissue, and can employ these as 

 building materials; — in a word, the collenchyma is capable of further growth. 

 This explains the advantage of collenchyma over hard bast cells, and woody fibres 

 or libriform cells. The hard bast and libriform cells when once fully formed lose 

 their capacity of further development, and would therefore be of little use as 



