474 MECHANISMS FOE CONVEYANCE TO AND FRO. 



the different structures taking part in the conduction are mutually of use to one 

 another, and are protected and saved from injurious external influences by the same 

 arrangement. 



The vessels and cells whose task is to conduct water and salts become lignified, 

 and the massive bodies of wood which exist in the trunks of old woody plants form 

 such a firm support that the thin-walled soft bast, when it clings to these and runs 

 parallel with them, is excellently protected from breaking. In those organs which 

 require to resist bending, however — in leaf-ribs and leaf-stalks, culms, and young 

 branches, — hard bast is put in as an accompaniment of the cells and tubes which 

 conduct up and down. These strands of thick-walled, but at the same time flexible 

 and elastic, cells of hard bast prevent the organs which they adjoin from being 

 broken and ruptured even under the influence of a considerable push and strain. 

 Let us look at the haulms, stems, branches, and leaves in a meadow or in a wood 

 during the sultry period which precedes the outburst of a storm. Not a leaflet 

 stirs, even the supple haulms are still, and every part of the plant, that true child 

 of light, assumes that position with regard to the light most beneficial to it. The 

 storm bursts, the wind whistles over the meadows, the leaves tremble, sway, and 

 flutter, the leaf -stalks shake, the culms bow and bend, the stems and branches are 

 smitten and arched so that their tops almost touch the ground; the foliage is 

 pelted with the rain, and shaken and displaced by every drop that falls on it. An 

 hour later the storm is over; here and there perhaps may be still seen a group of 

 stems and leaves prostrate under the weight of the rain-drops, and part of some 

 herbaceous stem which has been shaken by the storm bent like a bow, but the rest 

 stand again erect and motionless, as if they had never been disturbed by a breeze; 

 finally, even the plants bent by the shock and so severely prostrated by the rain- 

 drops raise their branches and foliage, and everything again resumes the same 

 conditions and position as before the outbreak of the tempest. But this is only 

 rendered possible by the presence everywhere of the elastic flexible strands of 

 hard bast accompanying the sieve-tubes and the other soft and delicate structures 

 which take part in the preparation and transportation of the organic materials. It 

 is indeed unavoidable that the cross section of the cells and vessels should become 

 narrowed in consequence of the push and strain caused by the gusts of wind, and 

 that, for example, the cross section of a cylindrical tube should become elliptical in 

 consequence of the curvature; but since the prostrated stem or leaf again rebounds 

 into the former position by reason of the elasticity of the hard bast, the alteration 

 produced by the push and strain is only temporary, necessitates no interruption 

 of function, is perhaps even beneficial to the movement of the materials, and, which 

 is the main point, no rupturing and no permanent bending of the soft delicate- 

 walled structures ensues. 



These delicate- walled elements, especially those of the soft bast, are protected 

 against harm from lateral pressure by the deposition of various tissues, especially 

 cork, in front of them (fig. 125^), which, like the buffers of an engine, keep off, or 

 considerably weaken, the lateral thrust and pressure. Remarkable contrivances for 



