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Show 128 SENSITIVENESS OF COTYLEDONS. CHAP II. the habit of cotyledons rising vertically at night or going to sleep, and their sensitiveness, especially that of their pulvini, to a touch; for all the al>ove~namcd plants sleep at night. On the oth.er hand, there are many plants the cotyledons of wh10h sleep, and are not in the least sensitive. As the cotyledons of several species of Cassia are easily affected. both by slightly diminished light and. ~y contact,. we thought that these two kinds of sensitiveness might be connected; but this is not necessarily the case, for tho cotyledons of Oxalis sensitiva did not rise when k?pt on one occasion for 1 ~ h., and on a second occaswn for nearly 4 h., in a dark closet. Some other cotyledons, as those of Githago segetum, are much affected by a feeble light, but do not move when scrat~hecl by a needle. That with the same plant there IS some relation between the sensitiveness of its cotyledons and leaves seems highly probable, for the above described Smithia and Oxalis have been called sensitiva, owing to their leaves being sensitive ; and though. ~he leaves of the several species of Cassia are not sensitive to a touch, yet if a branch be shaken or syringed with water, they partially assume their nocturnal dependent position. But the relation between the. sen· sitiveness to contact of the cotyledons and of the leaves of the same plant is not very close, as may. be inferred from the cotyledons of Mimosa pudica bemg only slightly sensitive, whilst the leaves ar~ well known to be so in the highest degree. Agam, the ]eaves of Neptunia oleracea are very sensitive to .a touch, whilst the cotyledons do not appear to besom any degree. CuAP. III. SENSITIVENESS OF RADICLES. 129 CHAPTER III. SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND '1'0 OTHEU luurr ANTS, Manner in which radicles bend when they encounter an obstacle in the soil-Vi cia fa. La, tips of rn.dicles l1ighly sensitive to contact and other irritants-Effects of too high a tempern.ture-Powcr of discriminating between objects attached on oppol:!ite sides-Tips of secondary radicles sensitivo-Pisum, tips of radicles sensitiveEffects of such sensitiveness in overcoming geotropism-Secondary radicles-Plmscolus, tips of radicles hardly sensitive to coutact, but highly senl:!itive to caustic and to the removal of a slice-Troproo~ u.m-Gossypium-Oucurbita-Ro.phn.nus-lEsculus, tip not sens1t1ve to slight contact, highly sensitive to cnustic-Qucrcus tip highly sensitive to contact-Power of discrimination-Zeu' tip highly sensitive, secondary l'adicles-Sensitiveness of radicle~ to moist air-Summary of chapter. IN order to see how the radicles of seedlings would pass o~er stones, roots, and other obstacles, which they must mcessantly encounter in the soil, germinating bea~s ( Vicia jaba) were so placed that the tips of the radicle~ came into contact, almost rectangularly or at a high angle, with underlying plates of glass. In other cases the beans were turned about whilst their radi?les were growing, so that they descended nearly vertically on their own smooth, almost flat, broad upper surfac~s. The delicate root~cap, when it first touched any duectly opposing surface, was a little flattened ~ransversely; the flattening soon became oblique, and ~n a few .hours quite disappeared, the apex now pointIng at right angles, or at nearly right angles, to its ~ormer co~rse.. The radicle then seemed to glide in ts new dnection over the surface which had opposed K • t'" : il:r t '"' :. ;)I ,.,, f::·:l |