8i8 
MECHANICS OF GROWTH. 
Normal plant 
in the light. 
Etiolated plant 
in the dark. 
Mean temperature. 
April 3 
3*8 mm. 
14-0 mm. 
9-4° C. 
4 
2'0 
13-8 
IO-6 
5 
11-^ 
io"7 
6 
0-7 
8-8 
I I'O 
7 
O'O 
4*4 
I I'O 
8 
2-1 
1 I'2 
9 
0-6 
11*5 
lO 
O'O 
125 
An internode of Humulus Lupulus gave — 
April 
22 
23 
24 
25 
26 
Increase in length 
in 24 hours. 
i9'o mm. 
25'0 
260 
17-2 
4-8 
Mean 
temperature. 
1 4-9'^ c. 
14-5 
143 
13-9 
i4'i 
Harting found that a Hop-stem, consisting of a number of internodes, which was 
492 millimetres long on May 15th had attained by the end of August a length of 
7'263 metres, this growth being distributed as follows over the different months: — 
0*492 metres in April. 
2*230 May. 
2*722 June. 
1*767 July. 
0052 August. 
These observations and a number of others show that the grand period of 
growth manifests itself even when the course of the changes of temperature acts in 
opposition to it ; i. e. when the temperature rises while the rapidity of growth de- 
creases owing to internal causes, and vice versa. The course of growth may no doubt 
be so modified by great changes of temperature that the curve of the grand period 
can no longer be recognised in the measurements. 
In order to determine the grand period of growth in a piece of a growing root, 
internode, or leaf-stalk, it is sufficient to mark a zone of the organ at the part where 
elongation is beginning by two lines of Indian ink, and to measure the daily (or 
half daily) growth of this piece until it ceases. 
By applying this method to the primary root of Vicia Faha, the temperature 
varying each day between 18° and 21*5° C, I found the following increase to take 
place in each period of twenty- four hours in a piece originally i mm. long situated 
immediately above the punctum vegeiationis : — 
ist day 
2nd 
3rd 
4th 
I '8 mm. 
3-7 
17-5 
i6'5 
