INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



287 



direct sunlight, the palisade parenchyma being weakly or not at all developed 

 in the former, while it is strongly developed in the latter (Fig. 135). Sunshine 

 leaves have smaller epidermal cells with smooth lateral walls, while shade leaves 

 have larger epidermal cells with wrinkled or wavy walls. These differences in 

 the epidermal cells, between leaves grown in sunshine and those grown in 

 shade, are so great that the two kinds of leaves might easily be regarded as 

 belonging to entirely different species (Fig. 136). 



In some cases very differently shaped leaves may be produced on the same 

 individual plant by allowing some leaves to 

 develop in sunshine and others in shade. 

 Campanula rotundifolia may serve to illus- 

 trate this (Fig. 137). This plant usually 

 produces two kinds of leaves: those near the 

 base (which develop in spring, in the shade 

 of surrounding plants) are rounded, kidney- 

 shaped and borne on long petioles, while 

 those on the upper part of the stem (which 

 develop later, in strong light) are linear, 



pointed at base and apex, and without long petioles. If a plant bearing both 

 sorts of leaves is kept for a time in very weak light the lateral buds on the 

 upper part of the stem develop reniform, long-petioled leaves, like those nor- 

 mally occurring exclusively near the ground. 



Although light is necessary for the normal development of green plants, they 

 do not necessarily develop normally with continuous illumination ; an alteration 

 of periods of light and darkness seems necessary to produce structures such as 

 occur in nature. Continuous illumination was obtained in the experiments of 



Fig. 135. — Cross-sections through 

 leaves of Fragaria vesca, grown in direct 

 sunlight (L), and in shade (5). (After 

 Dufour.) 



Fig. 136. — Surface view of upper leaf epidermis of Tussilago farfara, grown in direct sunlight 

 (L), and in shade (5). (After Dufour.) 



Bonnier 1 by means of electric arcs, the plants receiving no light but electric light 

 through the entire six or seven months of their development. Some of these 

 plants were lighted continuously, day and night, and others were darkened 

 by means of opaque covers, for a period each day from 6 p.m. to 6 a.m. The 

 injurious effect of ultra-violet light (which is relatively more intense in the 

 light of the electric arc than in sunlight) was avoided by the use of clear 

 glass screens, which of course absorbed the ultra-violet rays. 



In these experiments, the plants that were darkened at night developed in 

 the normal way and possessed normally differentiated tissues, but the con- 



1 Bonnier, Gaston, Influence de la lumiere electrique continue sur la forme et la structure des plantos. 

 Rev. gen. bot. 7: 241-257, 280-306, 332-342. 400-419. 1895. 



