for the following circumstance.  In the beginning of October, when the temperature was considerably higher, viz., 12o to 13o C., we found that only a few of the ungreased radicles became bent towards the sieve; and this indicates that sensitiveness to moisture in the air is increased by a low temperature, as we have seen with the radicles of Vicia faba relatively to objects attached to their tips.  But in the present instance it is possible that a difference in the dryness [page 185] of the air may have caused the difference in the results at the two periods.]

Finally, the facts just given with respect to Phaseolus multiflorus, Vicia faba, and Avena sativa show, as it seems to us, that a layer of grease spread for a length of 1 ½ to 2 mm. over the tip of the radicle, or the destruction of the tip by caustic, greatly lessens or quite annuls in the upper and exposed part the power of bending towards a neighbouring source of moisture.  We should bear in mind that the part which bends most, lies at some little distance above the greased or cauterised tip; and that the rapid growth of this part, proves that it has not been injured by the tips having been thus treated.  In those cases in which the radicles with greased tips became curved, it is possible that the layer of grease was not sufficiently thick wholly to exclude moisture, or that a sufficient length was not thus protected, or, in the case of the caustic, not destroyed.  When radicles with greased tips are left to grow for several days in damp air, the grease is drawn out into the finest reticulated threads and dots, with narrow portions of the surface left clean.  Such portions would, it is probable, be able to absorb moisture, and thus we can account for several of the radicles with greased tips having become curved towards the sieve after an interval of one or two days.  On the whole, we may infer that sensitiveness to a difference in the amount of moisture in the air on the two sides of a radicle resides in the tip, which transmits some influence to the upper part, causing it to bend towards the source of moisture.  Consequently, the movement is the reverse of that caused by objects attached to one side of the tip, or by a thin slice being cut off, or by being slightly cauterised.  In a future chapter it will be shown that sensitiveness to the attraction of [page 186] gravity likewise resides in the tip; so that it is the tip which excites the adjoining parts of a horizontally extended radicle to bend towards the centre of the earth.

Secondary radicles becoming vertically geotropic by the destruction or injury of the terminal part of the primary radicle.