The 11 Xerophytic ” Character of the Gymnosperms. 49 
capacities for water transportation in various woods. It is found 
that the amount of flow in vessels of the same length is roughly 
proportional to the square of the radius of the vessels. Hence the 
advantage in large wood vessels. 
To abstract some figures from Ewart’s 1 tables :— 
Plant. 
Rate of Flow 
in centimetres 
per hour. 
Internal radius 
of 
larger tracheae. 
Squares of 
Internal 
radius, x 10 8 
Relative volume 
passing through 
each trachea. 
Yew 
26 
0-0006 cm. 
36 cm. 
28 cm. 
B. Currant 
58 
0-0025 „ 
635 „ 
210 „ 
Elm 
50 
0-0033 „ 
1,099 „ 
400 „ 
Elder 
155 
0.0034 „ 
1,156 „ 
1,200 „ 
Marrow ... 
1,180 
0-0200 „ 
40,000 „ 
90,000 „ 
And when dealing with the average radius of the vessels and 
the maximal rate of the transpiration current we get:— 
Plant. 
Rate of Transpiration 
Current. 
Average Radius of 
Vessels x 10 4 . 
Yew 
... 16 cm. 
6 cm. 
Apple 
... 92 „ ... 
18 „ 
B. Currant 
... 121 „ ... 
22 „ 
Elder 
... 130 „ ... 
24 „ 
Pear 
... 158 „ ... 
25 „ 
Furthermore the length of the tracheal elements has a certain 
influence on the flow, though not in a simple ratio. In this respect 
also the Gymnospermic wood stands low in the scale. 
Plant. 
Greatest Length of 
Tracheal Element in 
Centimetres. 
Average Length of 
Larger Elements in 
Centimetres. 
Yew 
0-5 
0-25 
Raspberry 
15-0 
12-0 
Pear 
25 
18 
Elder 
24 
21 
Apple 
34 
24 
Wych Elm 
48 
36 
Strasburger 2 found that in the Taxus and Tsuga, with which 
he experimented, in order to drive a current of water through 
the stem at the transpiration rate, a head of water was required 
several times the length of the stem, while in Acacia a head of 
12 cm. sufficed for a piece of stem 10 cm. long. 
These figures, which represent but a minute fraction of the 
1 Ewart, A. J. “ The Ascent of Water in Trees.” Phil. Trans. 
Roy. Soc., 1905. B., p. 52. 
2 Strasburger, E. Hist. Beitr. 111. “ Leitungsbahnen,” Jena, 1891, p. 779. 
