1908.] NATURAL SCIENCES OF PHILADELPHIA. 449 



THE DIRECTIVE INFLUENCE OF LIGHT ON THE GROWTH OF FOREST PLANTS. 

 BY JOHN W. HARSHBERGER, PH.D. 



It is a well-known fact that light exercises a directive influence upon 

 plants. This directive influence is called heliotropism, or photo- 

 tropism. When a plant is grown in the window of a room, so that it 

 is unequally illuminated, that is, more powerfully through the window, 

 its leaves and even its stem are turned toward the incident rays of 

 light. This is known as positive heliotropism. If the common English 

 ivy, Hedera helix, be grown in pots by a north window, so as to 

 emphasize better the difference in light intensities, in about four 

 weeks it will be apparent that the growing sprouts are bending toward 

 the inner part of the room, away from the stronger light. This reaction 

 is negative heliotropism. 



The growth of forest plants is largely a question of light relationship. 

 Foresters recognize this fact and group trees into those intolerant of 

 the shade and those that are tolerant. The herbaceous plants, like- 

 wise, are influenced by the light which filters through the crown of 

 leaves above. The herbaceous spring flora of the forest requires more 

 light than the relatively few^ plants which flower in the autumn require, 

 when the trees are covered with foliage. These facts, although they 

 can be proved experimentally, are not always demonstrable to the 

 uninitiated. One of the best illustrations that the writer has seen 

 is the directive influence of light upon the leaves, or fronds, of the hay- 

 scented fern, Dicksonia pilosiuscula (= Dennstoedtia punctilobvla) , 

 which is widely distributed on open hillsides from New Brunswick and 

 Ontario to Indiana and Minnesota, south to Alabama and Tennessee, 

 ascending to 1680 m. in Virginia. The stipes of this fern are pale 

 green and chaffless, covered with fine hairs, and the leaves (10 dm. 

 long, 12-20 cm. wide) are ovate-lanceolate, acute or acuminate, fre- 

 quently long attenuate, usually tri-pinnatifid, thin and delicate in 

 the woods, tougher, more inrolled and more erect in the sun; rachis and 

 under surface of blades glandular pubescent. The observations 

 which the writer wishes to record on the directive influence of light 

 upon the position of the fronds were made at Pocono Pines, Monroe 

 County, Pennsylvania, where this fern is one of the most abundant 

 species. As the photograph will show (PI. XXIV), the upper surfaces 



