Concluding remarks and summary of chapter.

We do not know whether it is a general rule with seedling plants that the illumination of the upper part determines the curvature of the lower part.  But as this occurred in the four species examined by us, belonging to such distinct families as the Gramineae, Cruciferae, and Chenopodeae, it is probably of common occurrence.  It can hardly fail to be of service to seedlings, by aiding them to find the shortest path from the buried seed to the light, on nearly the same principle that the eyes of most of the lower crawling animals are seated at the anterior ends of their bodies.  It is extremely doubtful whether with fully developed plants the illumination of one part ever affects the curvature of another part.  The summits of 5 young plants of Asparagus officinalis (varying in height between 1.1 and 2.7 inches, and consisting of several short internodes) were covered with caps of tin-foil from 0.3 to 0.35 inch in depth; and the lower uncovered parts became as much curved towards a lateral light, as were the free seedlings in the same pots.  Other seedlings of the same plant had their summits painted with Indian ink with the same negative result.  Pieces of blackened paper were gummed to the edges and over the blades of some leaves on young plants of Tropaeolum majus and Ranunculus ficaria; these were then placed in a box before a window, and the petioles of the protected leaves became curved towards the light, as much as those of the unprotected leaves.

The foregoing cases with respect to seedling plants have been fully described, not only because the transmission of any effect from light is a new physiological fact, but because we think it tends to modify somewhat the current views on heliotropic movements.  Until [page 485] lately such movements were believed to result simply from increased growth on the shaded side.  At present it is commonly admitted* that diminished light increases the turgescence of the cells, or the extensibility of the cell-walls, or of both together, on the shaded side, and that this is followed by increased growth.  But Pfeffer has shown that a difference in the turgescence on the two sides of a pulvinus,—­that is, an aggregate of small cells which have ceased to grow at an early age,—­is excited by a difference in the amount of light received by the two sides; and that movement is thus caused without being followed by increased growth on the more turgescent side.** All observers apparently believe that light acts directly on the part which bends, but we have seen with the above described seedlings that this is not the case.  Their lower halves were brightly illuminated for hours, and yet did not bend in the least towards the light, though this is the part which under ordinary circumstances bends the most.  It is a still more striking fact, that the faint illumination of a narrow stripe on one side of the upper part of the cotyledons of Phalaris determined the direction of the curvature of the lower part; so that this latter part did not bend towards the bright light by which it had been fully illuminated,