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Show 484 CONCLUDING REMARKS AND CHAP, IX. CONCLUDING REMARKS AND SUMMAlW OF CHAP'l'ETI. \Ve 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 tho Graminere, Cruciferm, and Chenopodem, it is probably of common occurrence. It can hardly fail to be of service to seedlings, by aiding them to fin l the shortest path from the b1uied 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 bodie . 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 be· tween 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 un· covered parts became as much curvecl 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 Tropreolum majus and Ranunculus .ficaria; these ~vere then placed in a box before a window; and the petwles of the protected leaves became curved towards the lio·ht as much as those of the unprotected leaves. oT h'e foregoing cases with respect to see ellm' g Pl ants have been fully described, not only because t~e tr~ns· mis ion of any effect from light is a new physwlogiCal fact, but b cause we think it tends to modify somewh~t the current vle'iv on heliotropic moYements. Until CHAP. IX. SUMMARY OF CIIAP'fER. 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 tho extensibility of the cell-walls, or of both together, on tho shaded siue, and that this is followed by increased growth. But Pfeffer has shown that a difference in the tm·gescence 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 move,. ment is thus caused \vithout being followeu by increased growth on the more turgescent side.t All observers apparently believe that light acts directly on the part which bonus, but we have seen with the above described seedlings that this is not the case. 'fheir lower halves were brightly illuminated for honrs, 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 tho cotyledons of Phalaris determined the direction of tho curvature of the lower part; so that this latter part did not bend towards the bright light by which it had been fully illuminated, * Emil Godlewski has given ('Bot. Zeilung,' 1879, Nos. 6-9) an excellent account (p. 120) of the present state of 1110 question. See also VinP-s iu 'Arbeiton des ~t. lust. in Wiirzhurg,' 187H, B. n. pp. 114-147. Hngo de Vries has recently publi::~hed a still more important article on this tiubjeot: 'Bot. Zeitung,' Dec. 19th and 26th, 1879. t 'Die Periodischen Bewegungen der Blattorgane,' 1875, pp. 7, 63, 12~, &c. Frank has also insisted (' Die N o.turlicho wii.gerechte llichtung von Pibnzontheilen,' 1870, p. 53) on the important part which the pulvini of tbe leaflets of compound leaves plo.y in placing tl1e leaflets in a proper pobition with respect to the light. '!'his holds good, especially witlt the loaves of climbiug plants, which uro carried into nll sorts of positions, ill-o.dapted for theaction of tho light. |