We therefore allowed the radicles of 12 beans to grow at a temperature of between 55o and 60o F. The result was that in every one of these cases (included in the above-described experiments) the radicle was deflected in the course of a few hours from the attached object. All the above recorded successful trials, and some others presently to be given, were made in a sitting-room at the temperatures just specified. It therefore appears that a temperature of about, or rather above, 70o F. destroys the sensitiveness of the radicles, either directly, or indirectly through abnormally accelerated growth; and this curious fact probably explains why Sachs, who expressly states that his beans were kept at a high temperature, failed to detect the sensitiveness of the apex of the radicle.
But other causes interfere with this sensibility. Eighteen radicles were tried with little squares of sanded card, some affixed with shellac and some with gum-water, during the few last days of 1878, and few first days of the next year. They were kept in a room at the proper temperature during the day, but were probably too cold at night, as there was a hard frost at the time. The radicles looked healthy but grew very slowly. The result was that only 6 out of the 18 were deflected from the attached cards, and this only to a slight degree and at a very slow rate. These radicles therefore presented a striking contrast with the 44 above described. On March 6th and 7th, when the temperature of the room varied between 53o and 59o F., eleven germinating beans were tried in the [page 143] same manner, and now every one of the radicles became curved away from the cards, though one was only slightly deflected. Some horticulturists believe that certain kinds of seeds will not germinate properly in the middle of the winter, although kept at a right temperature. If there really is any proper period for the germination of the bean, the feeble degree of sensibility of the above radicles may have resulted from the trial having been made in the middle of the winter, and not simply from the nights being too cold. Lastly, the radicles of four beans, which from some innate cause germinated later than all the others of the same lot, and which grew slowly though appearing healthy, were similarly tried, and even after 24 h. they were hardly at all deflected from the attached cards. We may therefore infer that any cause which renders the growth of the radicles either slower or more rapid than the normal rate, lessens or annuls the sensibility of their tips to contact. It deserves particular attention that when the attached objects failed to act, there was no bending of any kind, excepting Sachs' curvature. The force of our evidence would have been greatly weakened if occasionally, though rarely, the radicles had become curved in any direction independently of the attached objects. In the foregoing numbered paragraphs, however, it may be observed that the extreme tip sometimes becomes, after a considerable interval of time, abruptly curved towards the bit of card; but this is a totally distinct phenomenon, as will presently be explained.
Summary of the Results of the foregoing Experiments on the Radicles of Vicia faba.--Altogether little squares (about 1/20th of an inch), generally of sanded paper as stiff as thin card (between .15 and .20 mm. in thickness), sometimes of ordinary card, or little frag- [page 144] ments of very thin glass etc., were affixed at different times to one side of the conical tips of 55 radicles. The 11 last-mentioned cases, but not the preliminary ones, are here included. The squares, etc., were most commonly affixed with shellac, but in 19 cases with thick gum-water. When the latter was used, the squares were sometimes found, as previously stated, to be separated from the apex by a layer of thick fluid, so that there was no contact, and consequently no bending of the radicle; and such few cases were not recorded. But in every instance in which shellac was employed, unless the square fell off very soon, the result was recorded.