CHAPTER X. 



1 Hi: MECHANICAL CONSTRUCTION OF THE PLANT-BODY. 



The texture of any normally growing Plant is relatively firm and 

 clastic, so that it keeps its form, and after yielding to pressure tends 

 recover. Even young shoots show this; but it is a much more 

 prominent feature in older plants. Woody trunks, large leaf -stalks 

 of Palms, and old roots consist of hard masses of resistant tissue. 

 They also may yield to the pressure of the wind, and they recover 

 very perfectly after it is over. But if the limit of elastic recovery is 

 passed, any part, young or old, may be so damaged that it is of no 

 further use to the plant. A trunk may be shattered, a limb or a leaf 

 may be severed, and so lost ; or soft cells may be crushed between 

 harder tissues, as may be seen in any leaf -blade roughly folded at 

 too sharp an angle. To minimise such risks it is necessary that 

 plants shall be mechanically constructed so as to resist those stresses 

 and strains which are likely to befall them in their ordinary course 

 of life. 



Individual plants often attain large size. The Brown Tangles of 

 the colder oceans, such as Macrocystis, may be several hundred feet in 

 length, and are among the largest of living organisms. Their leathery 

 body i^ anchored to rocks, and buoyed up by sea-water, though exposed 

 to its currents and waves. To maintain the form and attachment of 

 so large a plant offers a quite considerable mechanical problem, which 

 i- -hired by other water-plants in proportion to their size. But the 

 quirements in the case of aerial plants are much more exacting, for 

 th re not buoyed up by a medium of high specific gravity. They 

 must be stiff and firm of texture. Forest trees grow upwards to a 

 height sometimes of 300 feet or more, and there hold aloft the dead 

 weight of branches, leave-, and fruits. Not only must this be done 

 in still air, but they must also be ready to resist successfully the impact 



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