THE DIFFERENT KINDS OF STEM. 29 



The tendrils of the vine and pea ate well known; let us 

 show the beauty of their mechanism. When the plants are 

 young they are put forth in a straight line, and curved into a 

 sort of hook at their extremity. In this manner they seem as 

 if they were reaching forward for the purpose of catching hold 

 of something on which they can hang for support. If in this 

 state a young twig or branch be borne by the passing breeze 

 within reach of their h,ook, they immediately catch and coil 

 themselves spirally about it. Now this apparently feeble 

 organ of self-support is in reality a powerful instrument of self- 

 defence, and the storm which can overpower the strength of 

 the forest trees, prostrating them with the earth as it rushes 

 by in all the wildness of its fury, cannot injure these plants. 

 It is rendered harmless in its eiFects by the elastic yielding of 

 the tendril, which thus secures these weak plants from being 

 broken off from the object to which they have attached them- 

 selves, and from sustaining the slightest injury. 



In the grasses the stem, which is hollow and fistular, has 

 received the name of culm (culmus a straw). This structure 

 ' also prevails in other plants, and is a beautiful instance of 

 mechanical contrivance to dispose the limited quantity of 

 matter in the stem to the greatest possible advantage, so as to 

 give the greatest strength with4he least expenditure of mate- 

 rial. By this hollow cylindrical disposition of the matter, an 

 increase of strength is imparted to the vegetable structure 

 equivalent to that of a solid stem of the same diameter. The 

 bones of animals and the feathers of birds are tubes or hollow 

 trunks, combining strength with lightness, and constructed on 

 the same principle. 



When the philosopher Galileo was confined in the dungeons 

 of the Inquisition for teaching the heresy of the motion of the 



