EESISTANCE OF FOLIAGE-STKMS TO STRAIN, PRESSURE, AND BENDING. 725 



of material, also finds expression in the construction of the stem. In one case we 

 are reminded of the system of tubular bridges, in other cases of that of lattice- 

 bridges; here of a massive piUar-like structure with architrave and flattened top, 

 there of a Gothic building with pointed arches, buttresses, and steep gables; but 

 the special conditions of the habitat are always taken into consideration, and 

 the whole structure for this reason always exhibits the greatest adaptability of 

 the means to the end. 



The framework which gives the desired strength to the whole structure is 

 made up of parts which would be called by builders "constructive pieces", and 

 these are in turn made up of special cells, termed mechanical cells. Mechanical 

 cells have already been alluded to in the description of the conducting-apparatus 

 (cf. p. 474), although only very briefly. It was pointed out that the tubes and 

 cells which serve for the transport of fluid materials up and down the plant are 

 usually united into a bundle, the so-called vascular or conducting bundle, and 

 that when the constituent parts of this bundle occur in organs which are 

 exposed to the danger of being broken, mechanical cells always make their 

 appearance alongside the conducting cells and vessels. The delicate vascular 

 bundles then usually lie embedded in a channel of hard bast, or are protected 

 laterally by a strand of hard bast, or more rarely they are interposed between 

 two bands of this tissue. These strands and bands of hard bast are frequently 

 of merely local importance for the vascular bundle, and may be likened to the 

 strengthening appliances of gas and water-pipes in human dwellings, which are 

 very important in their special use, but do not help to strengthen the whole 

 house. Very often, however, these special supporting agents of vascular bundles 

 are absent, and then the conducting tissues are affixed to the groups of mechanical 

 cells which form the foundation-framework of the whole structure. 



The hard bast is the mechanical tissue most often employed in both cases. To 

 the naked eye the cells of hard bast look like tiny threads. They are elongated, 

 fusiform, pointed at both ends, and interlaced and dovetailed with one another as 

 shown in fig. 125 * (p. 469). They are generally about 1-2 mm. long, but in certain 

 cases attain a much greater length; those of the Hemp are 10, those of Flax 20-40, 

 of the Nettle 77, and of Boehmeria nivea even 220 mm. long. The walls of hard 

 bast cells are always very much thickened, and the cell-cavity is very narrow, often 

 being reduced to an exceedingly fine canal, in some cases, e.g. in the cells of hard 

 bast of Gorchorus olitorius (known as Jute), the canal here and there is quite 

 obliterated, so that the cell is transformed into a solid fibre. It is concluded from 

 the direction of pores which sometimes appear in the walls that the micellae which 

 build up the walls of these thick bast-cells are arranged in left-handed spiral lines, 

 and this spiral torsion is supposed to be connected with the strength of the whole 

 hard bast cell. It is known that bundles of straight threads are not as strong as 

 bundles twisted into a string, and we are justified in supposing that this is also the 

 case with the rows of micellae forming the extremely fine fibrillse in the walls of 

 the hard bast cell. When a cell of hard bast is fully developed, the living proto- 



