426 ARRANGEMENTS FOR RETAINING THE POSITION ASSUMED. 



fresh leaves ha'/e only short stalks, and these become so strong and elastic that 

 they are well able to support the leaves. Water-lilies are striking examples of 

 this. In Polygonum amphihium, the long stems of the aquatic form, bearing at 

 their upper ends groups of floating leaves, are much thinner than the short stems 

 of the terrestrial forms, which are uniformly beset with leaves from top to bottom. 



The green tissues which are surrounded with air are much more exposed to 

 the danger of being torn, bent, and broken up by violent gusts of wind than those 

 which live either wholly or partially submerged in water. 



When green tissue is only developed in the cortex of the branches, as in the 

 leafless switch-plants, the branches are always elastic and supple, and in order to 

 produce this quality, bundles and strands of hard bast, i.e. elongated spindle- 

 shaped thick-walled cells of fibrous appearance, are inserted at suitable places. 

 The wood in these branches is also very tough, and gusts of wind can consequently 

 do them little harm. They are often prostrated by storms; but when the wind 

 subsides, the branches forthwith rise up, and in consequence of their elasticity, 

 resume their former position towards the light. The bundles of hard bast-cells 

 alternate in many instances regularly with the green tissue, as, for example, in 

 Spartium scoparium, illustrated in fig. 81, and, generally, very manifold con- 

 trivances are to be found in the internal construction of the branches for 

 hindering the bending up and crushing of the green tissue. 



Leaves, as well as stems and branches, have originally a tendency to grow up 

 perpendicularly in the atmosphere, and there are many plants whose foliage 

 remains throughout life in this position. Obviously these leaves are no less 

 exposed to damage by storms than are the upright branches of the switch-plants. 

 It must be borne in mind that gusts of wind rush over the ground in waves like 

 a powerful torrent, and that the direction of the air-current is usually parallel 

 to the surface of the earth. Plant-organs which grow up from the ground are 

 struck at right angles by such gusts, and are thus exposed to the most violent 

 attacks of the wind. Leaves, especially, whose blades are inclined at right angles 

 to the direction of the storm, are much more easily bent and crushed than those 

 whose blades lie parallel to the current. The eft'ect of the attacks of wind increases 

 in proportion to the extent of the surface exposed to the air-current, and a large 

 upright projecting leaf will be bent much more by the wind than a small leaflet 

 which lies close to the stem like a scale. 



In what way can the dangers of rupture be v/arded off from a green leaf which 

 grows towards the light of heaven, is surrounded by air, and is exposed on all 

 sides to the attack of wind? First of all, at any rate, by the same developments 

 as those mentioned in the case of the upright green branches of switch plants, 

 i.e. a suitable placing of the green tissue between flexible, elastic, fibrous bundles 

 of bast-cells, by support from thick-walled woody cells, and other cellular forma- 

 tions; by these means firmness is given to the whole structure with the least 

 possible expenditure of material; an arrangement which the thin-walled green 

 tissue can, by itself, never have on account of its special function. 



