Sunshine, Rain, and Wind 



wind could scarcely amount to 490 gr. (about i lb.), so, 

 when taking into account the reduction of the leaf 

 surface, it seems unlikely that even the worst hurricanes 

 could tear away its leaves. 



In a general way, mechanical tissue is at once pro- 

 duced wherever strains or stresses occur. This is 

 perhaps most easily seen by looking at the ribs and 

 strings of ordinary herbaceous stalks and petioles. One 

 can follow these strings quite easily by cutting across 

 the stem, and it is then easy to see how exactly they 

 are placed where the strength is required. 



The strength of the wood in a tree and its branches 

 varies very much, but is always best developed where 

 the most strain is likely to occur. An exposed tree has 

 tougher and denser wood than one grown in close 

 forest. But if a forest tree is left by itself when its 

 companions are felled, its wood will (if it survives) 

 become tougher and more mechanically effective.^^ 



In leaf, stem, and root, the right sort of mechanical 

 tissue, whether for support as a buttress or to act 

 against a strong pull or tug, is always found in the 

 place where resistance or tenacity is wanted. Dr. 

 Wildt carried out some pretty experiments in the way 

 of stretching roots artificially by means of a weight 

 attached to a thread, which was passed over a pulley 

 and fastened to the root. He not only called into 

 existence mechanical tissues in these roots but altered 

 their anatomical structure.^^ 



So also, growing fruit-stalks develop mechanical strings 

 and cords as soon as the fruit becomes heavy and re- 

 quires support.^* This mechanical or strengthening 

 tissue depends for its efficiency both on the shape of the 

 elements composing it and upon their constitution. 



The tracheid, which is the ultimate unit in the forma- 

 tion of wood, is a marvel of engineering ingenuity, of 



190 



