374 MR. T. W. WOODHEAD ON THE 
will be of service in resisting strain in every direction. The 
stem is further strengthened by the development of well- 
marked stereom-strands between the steles (fig. 17). 
Bracken Leaf-stem. Oak-form. 
The leaf-stalk of a plant growing under moderate conditions 
of light, moisture, and wind in a Coal-Measure Oak wood shows 
in transverse section a somewhat cylindrical outline (fig. 19), 
flattened or slightly depressed, along the upper and lower 
surfaces. The sclerenchyma forms a firm but narrow band, 
often not more than four to five cells deep, with a slight 
tendency to increase along the upper surface (fig. 20). The 
supporting mechanism is that of a pillar or strut, and the great 
majority of specimens examined showed very little specialization 
beyond this, even in cases of very large fronds with leaf-stalks 
3 in. or more in diameter. In these the tendency is to develop 
one or more fairly large lacune (figs. 21 & 22). 
Leaf-stalk. | Shade-form. 
If we now examine plants growing in the deep shade of Elm 
or Beech, we find the leaf-stalk strikingly reduced, often less 
than å in. in diameter. Relatively large, very thin fronds are 
developed, but the leaf-stalk is too weak to support the weight 
of the blade, and the general habit of the plant is drooping or 
more or less prostrate. Fig. 23 is a transverse section of the 
leaf-stalk of a young frond, which had grown about a foot above 
the surface of the ground and before the blade had expanded. 
The outline is somewhat cireular, but with a distinet tendency 
to flatten on the upper side. The sclerenchyma is slightly 
developed peripherally, forming a very narrow band two or 
three cells deep (fig. 24). As the leaf-stalk matures and the 
frond unfolds, it will be under the stimulus of slight but con- 
tinuous stress, and a comparison of this with the mature form 
shows striking changes to have taken place (figs. 25 & 26). 
The outline becomes less cireular, and a well-marked girder- 
form is developed ; the tension-flange is narrow but considerably 
thickened, with a distinct wedge of sclerenchyma in the centre. 
The compression-flange is broadened considerably and develops 
lateral wings, which not only serve to withstand compression, 
but stiffen it for resisting lateral stresses, just as girders are 
