450 PROCEEDINGS OF THE ACADEMY OF [AugUSt, 



of the leaves are turned toward the hght, if the ilhimination is one- 

 sided. If the ilhimination is from all sides of the fern clump, then 

 there is no particular direction in which the leaf-blades face. The 

 one-sided illumination is obtained when the ferns grow along the 

 edge of the woods, composed in the Pocono region of white pines, 

 white birches, black spruces, beeches and maples, which on account of 

 their dense crown cut off much of the light from behind and above, 

 so that such woods can be called appropriately dark woods. The 

 photograph shows how all the leaves of a single patch are turned out- 

 ward toward the open field adjoining the woods, in obedience to the 

 directive influence of the light, so that the leaves stand, row after row, 

 all facing in one direction. 



The second and more striking example of the directive influence of 

 light is illustrated by the hobble-hush, Viburnum lantanoides (= V. 

 alnifolium), a shrub which ranges from New Brunswick to North Caro- 

 lina, western New York and ]\Iichigan, but which does not occur in the 

 woods near the City of Philadelphia. In the dark pine woods on the 

 Pocono plateau this shrub is extremely abundant, and where the 

 woods are the densest, not only are all of the branches and the leaves 

 directed by the incident rays of light, but they show permanent 

 structural changes which are induced by the directive light influence. 

 It is known that light has a most notable influence in the determina- 

 tion of the external form of a large number of plants. The develop- 

 ment of certain tissues or organs on one side of the axis of a shoot, 

 and their suppression on other parts of the plant body, may be regu- 

 lated experimentally by means of the character of the illumination. 

 This development of tissues on one side of the axis is illustrated finely 

 in the branches of adult forest-grown specimens of the hobble-bush. 

 If we examine young shrubs of this plant, illustrated in PI. XXV, 

 fig. 7, we see that the branching system follows the method of a dicha- 

 sium. The leaves in such young bushes stand perfectly horizontal, 

 so as to receive the incident rays of light on the upper surface of the 

 blade, and so as to present their profile to the observer standing in 

 front of the plant. As fig. 6 shows, they arrange themselves, when 

 viewed from above, in the pattern of a leaf mosaic, so that none of the 

 leaves overshadow the others. Such plants merely show the directive 

 influence of the light on the leaves, without showing any characteristic 

 growth differences. The same influence of light is manifested in the 

 stoloniferous branches which strike root, and which give the common 

 names hobble-bush or trip-toe to the plant (fig. 8). These plagio- 

 tropous shoots are only formed in the shade. The diminished light 



