Smithia and Oxalis have been called sensitiva, owing to their leaves being sensitive; and though the 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 sensitiveness 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 being only slightly sensitive, whilst the leaves are well known to be so in the highest degree.  Again, the leaves of Neptunia oleracea are very sensitive to a touch, whilst the cotyledons do not appear to be so in any degree. [page 129]

CHAPTER III.

SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.

Manner in which radicles bend when they encounter an obstacle in the soil—­ Vicia faba, tips of radicles highly sensitive to contact and other irritants—­Effects of too high a temperature—­Power of discriminating between objects attached on opposite sides—­Tips of secondary radicles sensitive—­Pisum, tips of radicles sensitive—­Effects of such sensitiveness in overcoming geotropism—­Secondary radicles—­Phaseolus, tips of radicles hardly sensitive to contact, but highly sensitive to caustic and to the removal of a slice—­Tropaeolum—&s
hy;Gossypium—­Cucurbita—­Raphanus—­Aesculus, tip not sensitive to slight contact, highly sensitive to caustic—­Quercus, tip highly sensitive to contact—­Power of discrimination—­Zea, tip highly sensitive, secondary radicles—­Sensitiveness of radicles to moist air—­ Summary of chapter.

In order to see how the radicles of seedlings would pass over stones, roots, and other obstacles, which they must incessantly encounter in the soil, germinating beans (Vicia faba) were so placed that the tips of the radicles 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 radicles were growing, so that they descended nearly vertically on their own smooth, almost flat, broad upper surfaces.  The delicate root-cap, when it first touched any directly opposing surface, was a little flattened transversely; the flattening soon became oblique, and in a few hours quite disappeared, the apex now pointing at right angles, or at nearly right angles, to its former course.  The radicle then seemed to glide in its new direction over the surface which had opposed [page 130] it, pressing on it with very little force.  How far such abrupt changes in its former course are aided by the circumnutation of the tip must be left doubtful.  Thin slips of wood were cemented on more or less steeply inclined glass-plates, at right angles to the radicles which were gliding down them.  Straight lines had been painted along the growing terminal part of some of these radicles, before they met the opposing