that figured at C; but in one case the apex did not rub against the upper part of the radicle and thus remove the card; and it consequently made, owing [page 179] to the continued irritation from the card, two complete loops, that is, a helix of two spires; which afterwards became pressed closely together.  Then geotropism prevailed and caused the apex to grow perpendicularly downwards.  In another case, shown at (D), the apex

Fig. 69.  Zea mays:  radicles excited to bend away from the little squares of card attached to one side of their tips.

in making a second turn or spire, passed through the first loop, which was at first widely open, and in doing so knocked off the card; it then grew perpendicularly downwards, and thus tied itself into a knot, which soon became tight!

Secondary Radicles of Zea.—­A short time after the first radicle has appeared, others protrude from the [page 180] seed, but not laterally from the primary one.  Ten of these secondary radicles, which were directed obliquely downwards, were experimented on with very small squares of card attached with shellac to the lower sides of their tips.  If therefore the squares acted, the radicles would bend upwards in opposition to gravity.  The jar stood (protected from light) on a sand-bath, which varied between 76o and 82o F. After only 5 h. one appeared to be a little deflected from the square, and after 20 h. formed a loop.  Four others were considerably curved from the squares after 20 h., and three of them became hooked, with their tips pointing to the zenith,—­one after 29 h. and the two others after 44 h.  By this latter time a sixth radicle had become bent at a right angle from the side bearing the square.  Thus altogether six out of the ten secondary radicles were acted on, four not being affected.  There can, therefore, be no doubt that the tips of these secondary radicles are sensitive to slight contact, and that when thus excited they cause the upper part to bend from the touching object; but generally, as it appears, not in so short a time as in the case of the first-formed radicle.

Sensitiveness of the tip of the radicle to moist air.

Sachs made the interesting discovery, a few years ago, that the radicles of many seedling plants bend towards an adjoining damp surface.* We shall here endeavour to show that this peculiar form of sensitiveness resides in their tips.  The movement is directly the reverse of that excited by the irritants hitherto considered, which cause the growing part of the

* ‘Arbeiten des Bot.  Institut., in Würzburg,’ vol. i. 1872, p. 209. [page 181]