OCR Text |
Show 134 SENSITIVENESS OF THE APEX CIIAP. III. always fixed to the cork-lids, for the convenience of manipulation, with the edge from which the radicle and plumule protrudes, outwards ; and it must bo remembered that owing to what we have called Sachs' curvature, the radicles, instead of growing perpendicularly downwards, often bend somewhat, even as much Fig. 65. A. B. c. Vicia faba: A, radicle beginning to bend from the att.ached little sqnarc o~ card! ~~ bent at a rectangle; C, bent into a circle or loop, with the tlp begmmng to bend downwards through the action of geotropism. as about 45° inwards, or under the suspended bean. Therefore when a square of card was fixed to the apex in front, the bowing induced by it coincided with Sachs' cu:vature, and could be distinguished. from it only by be~ng more strongly pronounced or by occurring more qmckly. To avoid this source of doubt, the squares CnAP. III. OF THE RADICLE OF TilE BEAN. 135 were fixed either behind, causing a curvature in direct opposition to that of Sachs', or more commonly to the right or left sides. For tho sake of brevity, we will speak of the bits of card, &c., as fix cl in front, or behind, or laterally. As the chief curvature of the radicle is at a little distance from the apex, and as the extreme terminal and basal portions are nearly straight, it is possible to estimate in a rough manner the amount of curvature by an anglo ; ancl when it is said that the radicle became defl cted at any angle from the perpendicular, this implies that the apex '"as turned upwards by so many degrees from the downward direction which it would naturally have followed, and to the side opposite to that to which the card was affixed. That the reader may have a clear idea of the kind of movement excited by tho bits of attached card, we append here accurate sketches of three germinating beans thus treated, and s looted out of several specimens to show the gradations in the degrees of curvature. We will now give in detail a series of experiments, and afterwards a summary of the results. In the first 12 trials, little squares or oblonas of sanded card 1·8 m~. in.length, and 1·5 or only 0·9 mrn. i~ breadth (i.e. ·07i of an mch m length and ·059 or ·035 of an inch in breadth) were ~ed with s.hellac to the tips of the radicles. In the subsequent als the httle squares were only occasionally measured, but were of about the same size. . (1.) A young radicle, 4 mm. in length, had a card fixed behind: after 9 h. deflected in the plane in which the bean is ~atte~e~, 50° from tho perpendicular and from the card, and in PPOBltiOn. to Sachs' curvature : no change next morning, 23 h. from the time of attachment. d (2.) Radicle 5·5 mm. in length card fixed behind· after 9 h efdle cted i n th e P1 a ne of the bean' 20° from the per·p endi.C ular· =hfrom the card, and in opposition to Sachs' curvature: after ·no change. |