* ‘Arbeiten Bot.  Instit., Würzburg,’ Heft, iv. 1874, p. 620.

** Ibid.  Heft iii. 1873, p. 437.

*** ‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25. [page 156]

inch square, or rather less) were attached in the same manner to one side of the radicle at a distance of 3 or 4 mm. above the apex.  In our first trial on 15 radicles no effect was produced.  In a second trial on the same number, three became abruptly curved (but only one strongly) towards the card within 24 h.  From these cases we may infer that the pressure from a bit of card affixed with shellac to one side above the apex, is hardly a sufficient irritant; but that it occasionally causes the radicle to bend like a tendril towards this side.

We next tried the effect of rubbing several radicles at a distance of 4 mm. from the apex for a few seconds with lunar caustic (nitrate of silver); and although the radicles had been wiped dry and the stick of caustic was dry, yet the part rubbed was much injured and a slight permanent depression was left.  In such cases the opposite side continues to grow, and the radicle necessarily becomes bent towards the injured side.  But when a point 4 mm. from the apex was momentarily touched with dry caustic, it was only faintly discoloured, and no permanent injury was caused.  This was shown by several radicles thus treated straightening themselves after one or two days; yet at first they became curved towards the touched side, as if they had been there subjected to slight continued pressure.  These cases deserve notice, because when one side of the apex was just touched with caustic, the radicle, as we have seen, curved itself in an opposite direction, that is, away from the touched side.

The radicle of the common pea at a point a little above the apex is rather more sensitive to continued pressure than that of the bean, and bends towards the pressed side.* We experimented on a variety (York-

* Sachs, ‘Arbeiten Bot.  Institut., Würzburg,’ Heft iii. p. 438. [page 157]

shire Hero) which has a much wrinkled tough skin, too large for the included cotyledons; so that out of 30 peas which had been soaked for 24 h. and allowed to germinate on damp sand, the radicles of three were unable to escape, and were crumpled up in a strange manner within the skin; four other radicles were abruptly bent round the edges of the ruptured skin against which they had pressed.  Such abnormalities would probably never, or very rarely, occur with forms developed in a state of nature and subjected to natural selection.  One of the four radicles just mentioned in doubling backwards came into contact with the pin by which the pea was fixed to the cork-lid; and now it bent at right angles round the pin, in a direction quite different from that of the first curvature due to contact with the ruptured skin; and it thus afforded a good illustration of the tendril-like sensitiveness of the radicle a little above the apex.