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Show 178 SENSITIVENESS OF TilE AP.EX CIIAP. III. on it. There were therefore only 10 out nf the 68 which certainly were not acted on. Some of the radicles which were experimented on were young aml short, most of them of moderate length, an<l two or three exceeded three inches in length. 'l'he curvature in the above cases occurred within 2-:l: h., but it was often conspicuous within a much shorter period. l!..,or instance, the terminal growing part of one radicle was bent upwards into a rectangle in 8 h. 15 m., and of another in 9 h. On one occasion a hook was formed in 9 h. Six of the radicles in a jar containing nine seeds, which stood on a sand-bath, raised to a temperature varying from 76° to 82° F., became hooked, and a seventh formed a compl te loop, when first looked at after 15 hours. The accompanying figures of four germinating seeds (Fig. 69) show, firstly, a radicle (A) the apex of which has become so much bent away from the attached square as to form a hook. Secondly (B), a hook converted through the continued irritation of the card, aided perhaps by geotropism, into an almost complete circle or loop. The tip in the act of forming a loop generally rubs against the upper part of the radicle, and pushes off the attached square ; the loop then contracts or closes, but never disappears; and the apex afterwar<ls grows vertically downwards, being no longer irritated by any attached object. 1'his frequently occurred, and is represented at C. 1'he jar above mentioned with the six hooked radicles and another jar were kept for two additional days, fo~ the sake of observino- how the hooks would be mocl1fied. b M:ost of them became converte<l into simple loops, like that figured at C ; but in one case the apex did not rub against the upper part of the radicle and t~us remvve the card; and it consequently made, owmg CHAP. III. OF THE RADICLE OF ZEA. 179 to the continued irritation from the car<1, 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 A B. .C D. Zea mays: radicles excited to bend away from the little squares of uml attached to one side of their tips. in k' fi ma mg a second turn or spire, passed throuO'h the dr s.t lo op, wh 1' c h was at first w.1 dely open, ab nd in ?Ing so knocked off the card; it then grew perpend~~ larly downwards, and thus tied itself into a knot, w tch soon became tight ! fi Second ary R a dw · le s of Zea.-A short time ·a fter the rst radicle has appeared, others protrude from the N :2 |