i88 THE BIOLOGY OF FLOWERING PLANTS 



that the rays of the sun pass through the foUage : the 

 needles of fir and spruce are flat and broader. The xero- 

 phytism of the conifers has already been related to their 

 winter green character. It is equally suited to life in 

 moderately arid conditions, as in Asia Minor and the 

 Mediterranean countries. It is likely that it is primarily 

 related to the fact that conifer wood is a much less effec- 

 tive conductor of water than that of the angiosperm, since 

 it possesses only tracheids, narrow and short compared 

 with the vessels of broad-leaved trees. The resistance to 

 water-flow is much greater. Farmer (19 19) has measured 

 the " specific conductivities " of a number of woods ; by 

 this he means the rate of flow of water through a standard 

 length and area of wood in unit time. He finds the specific 

 conductivity of the pine to be 13, of the larch 14, and of 

 the yew 12, while for the oak and the beech the figures are 

 75 and 65 respectively. 



This once more emphasises the fundamental point 

 that it is not only the amount of water available in the 

 soil that determines the plant's condition, but the relation 

 between the rate at which the supply can be drawn on and 

 the loss. In trees with so high a resistance to water-flow 

 it is probable that, with external conditions favourable to 

 supply, the internal resistance is frequently the limiting 

 factor. If we look on this as the factor primarily related 

 to possession of a xerophytic needle-leaf by the conifers, 

 we can readily see that these trees would thus be fitted to 

 maintain a winter green foliage, and also to form communities 

 in more or less arid situations. 



These considerations also apply to the conifers which 

 possess the scale type of leaf seen in Cupressus, Chamae- 

 cyparis, and Thuja. The leaves are small and adhere to 

 the stem over part of their surface ; only the tip is raised 

 and free. These types are characteristic of arid stations. 

 Examples of gymnosperms with expanded leaves are the 

 cycads, Agathis, and Welwitschia with leathery xerophytic 

 leaves, and Ginkgo, which is deciduous. 



The needle and cupressoid scale types of leaf are not 



