ARRANGEMENTS FOR RETAINING THE POSITION ASSUMED. 427 



But the whole shape and position of the leaf must also be adapted to the 

 circumstances, for the simple reason that a plant constructed unsuitably with 

 regard to the prevalent winds would suffer injury, perish, and sooner or later be 

 suoplanted by other species better adapted to the given conditions. Therefore, it 

 may so far be looked upon as an adaptation of form when a leaf lies with its 

 surface parallel or only slightly inclined to the surface of the earth, and therefore 

 to the direction of the wind, so that the moving currents strike it at a very 

 oblique angle, and rupture of the blade can hardly ensue. Since this position 

 of the green leaves is also very favourable for most plants with regard to light, 

 it is not surprising that it occurs so generally. In such flat leaves, a rising and 

 falling, and occasionally a bending of the blade is unavoidable, especially when 

 the gust of wind comes from that side towards which the tip of the leaf is turned. 

 But such an attack on the leaf -blade, which is parallel to the surface of the ground, 

 or inclined slightly to the horizon, is rendered as little injurious as possible by two 

 arrangements. 



One consists in the fact that the moderately stiff" leaf-blades can turn like 

 weathercocks on the stem from which they project; this occurs in many reed- 

 like grasses, particularly in Phalaris arundinacea, Eulalia JajDonica, and 

 in the widely-distributed Fhragmites comnnunis. The latter, which often grows 

 in immense quantity in the marshy lowlands, in the depth of valleys, and on the 

 banks of rivers, develops lofty, slender culms bearing numerous leaves. These 

 leaves, like all grass-leaves, consist of a linear, fairly broad, and tapering blade 

 projecting from the stem; and also of a sheath in the form of a hollow cylinder 

 surrounding the haulm, and from which the portion of the haulm in question 

 proceeds as from a tube. As long as the haulms and leaves are not fully developed,, 

 the leaf- blades are strongly directed in a line parallel to the culm; later, they 

 decline, project horizontally, and finally become even somewhat depressed, so that 

 their apices are directed groundwards. They then remain flat, and are so stiff that 

 ihey cannot be bent by light winds. Moreover, if a stronger gust occurs, they do 

 not bend, but twist round like the weathercock on the roof -gable in the direction 

 of the wind. This is rendered possible only by the fact that the haulm and the 

 tubular leaf-sheath surrounding it are very smooth at the surfaces in contact 

 with each other, and that the leaf -sheath may undergo a slight splitting without- 

 damage. 



This development is found in the reed-like grasses mentioned; and in them 

 there is a further contrivance, an interrupted membrane or flap inserted at the 

 boundary between the blade and sheath; this protects the sheath from the entrance 

 of rain-water, and consequent increase of friction, rendering the twisting difficult. 

 The common Reed (Phragmites communis), growing in quantities, presents a 

 characteristic appearance, in consequence of the arrangement here described, every 

 time a breeze passes over such a bed. If the wind blows from the east, all the 

 leaves are directed to the west; if it comes from the west, all their apices are 

 turned to the east. The whole mass looks as if it had been combed, as if all the- 



