264 Mr G. F. Scott Elliot on the 



II. Mechanical System. — Algse require a certain amount of 

 mechanical strength, and this is imparted by the swollen 

 condition of their cells, just as an india-rubber tube filled 

 with water at a high pressure has a considerable rigidity. 

 In higher plants a special tissue (stereom) is found, con- 

 sisting of bast {i.e., hard bast), the libriform cells (bast- 

 like cells of the wood), the collenchyma, and sclerenchyma. 

 As regards the different elements in this system — 

 (1) The hast and libriform cells show pores arranged in a left- 

 handed spiral, from which we infer that the fibrillse are 

 also arranged in a left-handed spiral, so that the cells may be 

 considered as a number of small hollow cables dovetailing 

 into one another through their fusiform shape. 



(2) The collenchyma cells contain living protoplasm, and 

 the thickening is laid down in the corners of the cells. 

 They are therefore specially adapted to growing tissues. It 

 is interesting to note that in tlie development of typical 

 hard bast, a collenchymatous thickening is laid down in the 

 corners of the cells which is afterwards reabsorbed. 



(3) Sclerenchyma cells, that is to say parenchymatous 

 mechanical elements, such as the stone cells of many fruits. 

 Their function in many cases is quite uncertain. The direct 

 researches of Schwendener and others, as to the mechanical 

 properties of hard bast, show an enormous capability of 

 bearing weight. In Pincenectia recur vata this is equal to 

 that of the best German steel. 



One chief object to be attained is a certain rigidity 

 against the bending force of the wind. And just as in a 

 bent piece of iron, the convex side is exposed to tension 

 and the concave to pressure, while there is a central 

 unstrained part; so in the xjlant one finds the bast elements 

 arranged in bundles towards the periphery, these bundles 

 being united across the centre by weaker parenchyma. 



In cylindrical organs {e.g., Lamium album) one finds 

 simply two cross girders, or a set of peripheral girders 

 {Scirpus coespitosus and especially Juncns glaucus). These 

 peripheral bundles may fuse into a hollow cylinder with the 

 conducting elements embedded in it. In ordinary leaves 

 {e.g., Iris), there is a simple longitudinal girder seen in 

 transverse section. In overhanging leaves {e.g., Typha), the 

 concave part of the girder is cylindrical, and the convex 



