INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



257 



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- p,^. i3s.-Cross-sections through 



shaped and borne on long petioles, while leaves of Fragaria vesca, grown in direct 



those on the upper part of the stem (which ^"«^^^' ^"^ ''^ '''^^' ^^^' ^^^'" 

 develop later, in strong Hght) 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 developiment of green plants, they 

 do not develop normally with continuous illunaination: an alteration of periods 

 of light and darkness is necessary to produce structures such as occur in nature. 

 Continuous illumination was obtained in the experiments of Bonnier^ by means 



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

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



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 



1 Bonnier, Gaston, Influence de la lumi^re filectrique continue sur la forme at la structure des plantes. 

 Rev. gto bot. 7 : 241-257, 289-306, 332-342, 409-419. 189S. 

 17 



