GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 787 
shorter, precisely in accordance with the phenomena described in paragraphs ^ and c^. 
For the measurements in the following table as thick shoots as possible were used, 
since they give considerable differences in length betw^een the convex and concave 
sides even when the curvature is slight. The measurements were made with strips 
of card graduated in millimetres, and which I applied closely to the concave and 
convex sides. 
Original 
Approximate 
Lengthening 
Contraction 
Name. 
radius of 
of the 
of the 
length. 
curvature. 
convex side. 
concave side. 
Silphium perfoliatum . 
152 mm. 
18 cm. 
3*4 p.c. 
o*o p.c. 
do. do. . 
120 
17 
•6 
Macleya cordata . 
87-5 
7 
2*3 
1*7 
do. do. . 
104 
24 
•5 
15 
Polygonum Fagopyrum 
63 
8 
2'I 
1-6 
Helianthus tuberosus 
98 
2'0 
Valeriana exaltata 
150 
32 
•8 
•7" 
do. do. . 
1 10 
•7 
2*1 
Vitts 'vinifera 
149 
6-10 
1-3 
2'0 
The permanent curvature which remains after violent oscillations of a shoot, or 
the Curvature of Concussion, is the result of a lengthening of the convex and a 
simultaneous shortening of the concave side. A proof is thus afforded that the 
whole phenomenon is dependent on the very imperfect elasticity and the great 
flexibility of the region that is capable of flexion 2. A shoot bent in this way shows 
the same changes as one that is simply bent between the hands. This result 
would not be at all altered were it found, in harmony with what was said in para- 
graph that the concave side was also sometimes slightly lengthened, since it is 
stretched by the recoil of the oscillations; and this elongation is not always entirely 
neutralised. Prillieux has compared this curvature to that of a lead-wire fixed to 
an elastic support, when the support was struck ; he was unable however to see 
the reason why the older and younger parts of the shoot did not exhibit the phe- 
nomenon. In the older parts this depends on their more perfect elasticity, in the 
younger on their smaller flexibility, and on the circumstance that they are not 
strongly bent, but are only thrown backwards and forwards by the oscillations of 
the lower and more flexible parts. 
The subsequent neutralisation of the curvature by growth must depend first of 
all on the increase of turgidity in the concave and its diminution in the convex 
side, and on the growth being consequently promoted in the former. This may 
be assisted also by the secondary effect of elasticity, in consequence of which the 
stretched epidermis of the convex side shortens, while the compressed tissues of 
the concave side expand. 
Sect. 14. — Causes of the condition of Tension in Plants. The elasticity 
of the organised parts of plants results in tension chiefly from the operation of three 
causes; viz. (i) the turgidity, in other words the hydrostatic pressure of the contents 
of the cell on the cell-wall; (2) the swelling and contraction of the cell- walls when 
^ According to Hofmeister all the sides of the shoot become longer. He calculated the length 
of the curve which he took for an arc of a circle ; and Prillieux measured only the concave side, which 
he found to be always shorter ; the contraction of the whole shoot, /. e. of its neutral axis, cannot 
however be inferred from that of the concave side. The thickening which, according to Hofmeister, 
should take place, if the shoot becomes longer on all sides, I consider cannot be demonstrated, in 
consequence of the extremely small change in diameter which takes place in such cases. 
^ Compare the different description given by Hofmeister in his paper On the Bending of the 
Succulent Parts of Plants, in the Berichte der kön. sächs. Ges. der Wiss,, 1859. 
3 E 2 
