Sunshine, Rain, and Wind 
wind could scarcely amount to 490 gr. (about 1 Ib.), 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.” 
Soalso, 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 
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