Secondly, with the radicles of
seedlings, the tip is sensitive to various stimuli, especially to very
slight pressure, and when thus excited, transmits an influence to the upper
part, causing it to bend from the pressed side. On the other hand, if the
tip is subjected to the vapour of water proceeding from one side, the upper
part of the radicle bends towards this side. Again it is the tip, as stated
by Ciesielski, though denied by others, which is sensitive to the
attraction of gravity, and by transmission causes the adjoining parts of
the radicle to bend towards the centre of the earth. These several cases of
the effects of contact, other irritants, vapour, light, and the
[page 5]
attraction of gravity being transmitted from the excited part for some
little distance along the organ in question, have an important bearing on
the theory of all such movements.
[Terminology.--A brief explanation of some terms which will be used, must
here be given. With seedlings, the stem which supports the cotyledons (i.e.
the organs which represent the first leaves) has been called by many
botanists the hypocotyledonous stem, but for brevity sake we will speak of
it merely as the hypocotyl: the stem immediately above the cotyledons will
be called the epicotyl or plumule.
Pages:
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26