ECOLOGY OF WOODLAND PLANTS. 975 
sometimes stiffened by side-pieces to resist lateral vibrations. 
In addition we find lateral stereom strands developed in the 
ground-tissue between the steles. These might be compared to 
the similar strands found in the exposed form. Fig. 26 shows 
these outer tissues with the much thickened stereom-band. 
Under these conditions and in such habitats, this form frequently 
recurs throughout the district. 
Frequently we find leaf-stalks of a similar diameter in the 
Bracken of an Oak wood, but they are erect, not drooping. 
They are subjected mainly to compressive stresses, and the 
tendency of the stalk is to form the strongest strut or pillar. A 
transverse section (fig. 27) of such a slender-stalked form, 
therefore, shows a distinct pillar-mechanism with the strengthen- 
ing material as far from the centre as possible. The outline is 
somewhat circular, with a uniform band of much-thickened 
sclerenchyma nine to ten cells deep (fig. 28), the fibres of which 
are more brittle than in the deep-shade form. In this the 
strands between the steles are often not developed. 
The Bracken thus affords an interesting example of the deve- 
lopment of mechanical tissues apparently as a result of tensile 
and compressive stresses. In the shade-form the stresses are 
small but continuous, due to the weight of the relatively large 
frond. Under opposite conditions in dry, open, windy situations, 
although the plants are dwarfed and thus relatively protected, 
the stalk is affected by varying stresses in all directions, and 
here we find the development of the stereom to be enormously 
increased. Under medium conditions, in the shelter of an Oak 
wood, the leaf-stalk is tall, erect, and pillar-like. Every grada- 
tion between these forms is found under intermediate conditions. 
Many of the specimens first examined showed these features to 
recur so often as to give support to Hegler’s results (46). 
But forms were found which introduced an element of un- 
certainty ; and it is obviously impossible to Judge of all the 
conditions influencing these structures in the field. The excep- 
tions were frequent enough to show that although the stimulus 
of stresses might be a contributory cause, evidently other 
influences were at work as yet undetermined. The evident 
pliability of this species recommends it as suitable for experiments 
in this direction, but the results are as yet too incomplete to be 
dealt with here. Bali (3), who has recently repeated Hegler’s 
observations, contends, as the result of numerous interesting 
