198 POPULAR SCIENCE MONTHLY. 



The variations in external form described above are due to the 

 intensity of the illumination. At the same time the structure and 

 arrangement of the cells depend on the direction from which the 

 light rays come. Thus, an organ receiving light from one side only 

 will exhibit a structure different from an organ of the same kind 

 receiving direct rays from two or more sides. Light, then, is a cause 

 of dorsiventrality — that is, of the fact that the upper and lower sides 

 of organs are not alike in structure. The leaf affords a splendid ex- 

 ample of dorsiventrality as a result of the exposure of one side only 

 to direct light. The upper side of a horizontal leaf, such as the oak, 

 beech, or maple, contains one or two layers of cylindrical cells with 

 their long axes perpendicular to the surface. In vertical leaves, such 

 as the iris, these palisade cells, as they are termed, are not so well 

 defined, and in all leaves grown in darkness this tissue is very much 

 reduced. If a young leaf not yet unfolded from the bud is fas- 

 tened in such a position that the under side is uppermost, pali- 

 sade cells will be formed on the side exposed to the direct rays of 

 the sun. 



The influence of light upon the sporophylls, or reproductive or- 

 gans of the seed-forming plants, is quite as well defined as upon the 

 vegetative organs. 



In general it is to be said that stamens and pistils may reach 

 functional maturity in darkness or diffuse light, and if pollination is 

 provided for, seed and fruit formation may ensue. 



The diminution of light has the effect of transforming inflores- 

 cences into leafy shoots in some instances, however. The more 

 common reaction consists of alterations in the size, form, and color 

 of the perianth, and greater changes are induced in the petals than 

 in the sepals. The corolla shows greater decrease in size in Melan- 

 dryum and Silene, in diffuse light, though the relative form is main- 

 tained. The writer has obtained most striking results from growing 

 flowers of Salvia (sage) in a dark chamber, inclosing the inflo- 

 rescence only. In the normal flower the irregular scarlet corolla 

 attains three times the length of the calyx, and two stamens extrude 

 from under the upper lip. When grown in darkness, the corolla 

 with the adherent stamens measure about three millimetres in length, 

 or one twelfth the normal, and are scarcely more than half the size 

 of the calyx, which is but two thirds the size of similar organs grown 

 in the light. The color is entirely lacking from the corolla, and is 

 found only along the veins of the calyx. 



In other instances in which the corolla is composed of separate 

 members, an unequal reaction is exhibited. The corolla of nastur- 

 tium (Tropwolum ma jus) consists of five approximately equal petals. 

 Flowers of this species grown in darkness show one of nearly normal 



