OCR Text |
Show 160 SENSITIVENESS OF THE APEX CHAP. III. of card attached to their tips. Firstly, 13 peas, most of them having very short and young radicles, were placed in an ice-box, in which the temperature rose during three days from 44° to 47° F. They grew slowly, but 10 out of the 13 became in the course of the three days very slightly curved from the squares; the other 3 were not affected; so that this temperature was too low for any high degree of sensitiveness or for much movement. Jars with 13 other radicles were next placed on a chimney-piece, where they were subjected to a temperature of between 68° and 72° F., and after 24 h., 4 were conspicuously curved from the cards, 2 slightly, and 7 not at all; so that this temperature was rather too high. Lastly, 12 radicles were subjected to a temperature varying between 72° and 85° F., and none of them were in the least affected by the squares. The above several trials, especially the first recorded one, ind.icate that the most favourable temperature for the sensitiveness of the radicle of the pea is about 60° F. The tips of 6 vertically dependent raclicles were touched once with dry caustic, in tho manner described under Vicia faba. After 24 h. four of them were bent from the side bearing a minute black mark ; and. the curvature increased in one case after 38 h., and in another case after 48 h., until the terminal part pro· jected almost horizontally. The two remaining ra· dicles were not affected. With raclicles of the bean, when extended horizontally in clamp air, geotropism always conquered the effects of the irritation caused by squares of card attached to the lower sides of their tips. A similar experim~n t was tried on 13 radicles of the pea; the squares bemg attached with shellac, and the temperature between 58°-60° F. The result was somewhat different; for CHAP. III. OF 'fHE RADICLE OF 'filE PEA. 1Gl these r~clicles are eit.hor less strongly acted on by geotropism, or, what IS more probable, are more sensitive. to conta~t. A~ter a time g otropism always prevail.ed, but Its actiOn was often delayed.; and. in three mstances .thoro was a mo. t curious struggle between geotropism and the irritation caused by the cards. Four of the 13 radiclcs were a little curved down~ards within 6 or 8 h., always reckoning from the time when the squares were first attaehecl, and after 23 h. three of them pointed vertically <lownwar~ s, and the fourth at an angle of 45° ben ath the honzon. These four radielcs therefore diu not seem Fig. 67. l- A. B. Pisum sativum ·. a rad J·C 1e extcnc 1c 1l hon.z ontally m. damp nir with a littl~ ~q~:::d of. card a~~ed to the lower side of its tip, causing it to bend aher 21 8 ~n oppositiOn to geotl'opism. The deflection of the radicle B f ou:s Is shown at A, and of the same radi cle after 45 hours nt 'now ormmg a loop. ~have been at all affected by the attached squares. thour others were not acted on by geotropism within de first 6 or 8 h., but after 23 h. were much bowed 2~'~n. b Two others remained almost horizontal for 1 ., ~t afterwards were acted on. So that in the e da tter SIX ca th . 1 ses e actwn of geotropism was much de ayed. The eleventh radicle was slightly curved th own aft~r 8 h., but when looked at a()'ain after 23 h. e termmal por t.w n w. as curved upwba rds; if it had M |